The simple answer is because of the fall of mankind. Human beings have suffered horribly since Adam and Eve trusted and believed the devil's words instead of God's. God alone has the words of life. Anything from the Enemy of our souls will be a lie.
Let's look at something from the Bible in Romans 1*. Most of us are familiar with this passage and this is one reason for mental illness. It starts with human beings not liking God or His standards. When we don't like something, we have wishful thinking. We wish really hard that whatever it is we don't like would go away. We eventually believe what we want to believe. But in Romans 1, God interacts with us as well. When we suppress the knowledge of God that God gives us, eventually, all we see is nothing of God. God will not continue to deal with our hearts once we have hardened our hearts against Him. He will not make us believe what is true. He will have left us to our own devices.
The result of this is a lack of normal mental processes. But not every person who is experiencing mental illness is a person described in Romans 1. Could there be another reason for mental illness besides suppressing the knowledge of God?
The mind is complex, and living in a fallen world with fallen people makes everything more complicated. But there are some things that are concrete, even though we might not have all the answers or know how to cure people of mental illness that comes about because of victimization.
Victimization is a scary word. When some hear the word, 'victimization', they automatically think of blame shifting. For example, have you ever heard the outcome of a court decision to let a person go free who has done a horrible crime, just because he or she was mentally ill? It can become a scapegoat for our sins and crimes against others if we are not careful and skilled in knowing how to deal with the mentally ill.
But there is a mental condition that is very, very prevalent today, and I think it needs attention. The reason it needs attention is because many are experiencing this type of condition, yet, many who would counsel them would assume that these people are in the Romans 1 category, when in reality their cause for mental illness is very different. The end product looks the same, but the cause is very different, and we need to treat the two types of mental illnesses differently.
When children are traumatized, they basically shut down. The conscious part of a person goes completely blank and the subconscious part takes over. It is a little like sleep walking. Many times, a person's emotions are gone, as well as a conscience. Some of these people seem like robots. There is a valid reason for this; their subconscious is protecting them from being hurt again, or from potentially going through another traumatic situation.
The statistics of women who have been raped as a child are alarming. What is even worse, is that being raped is like a check-mate. A woman who has been raped not only has the disgrace of being raped, but, the rapist usually will put the blame of the rape onto the woman, and, he will threaten the woman with harm, should she report or tell of the rape. If that isn't bad enough, many women have wanted to tell of the rape, but their stories are not believed.
Some women have told and have suffered badly. One woman I know told her mother about what her father did to her. Instead of the mom listening to her daughter, she beat her severely. The more trauma a child suffers, the more difficult his or her mental illnesses will be.
A common effect of this particular mental illness is the appearance of not having a conscience. People can do things in public that they normally would never do. Most of the time, when confronted later on about the episode, they will not be able to recall it.
How can a person go into a building or a bus, and blow it up, and not feel any concern that they will be killing many people along with themselves. It is true that some religions, such as Islam, would teach that there is reward for killing certain people of particular groups, such as Christians. They believe they will have Allah's favor on them if they kill those whom they deem as Infidels, but that still doesn't answer the question of why there doesn't seem to be a conscience or concern.
When people are about to die, many are very concerned about what happens next, but those who blow up buildings and buses aren't. Why do you think this is? Drugs? Possibly. But it is likely that the person doing the blowing up has been mentally destroyed by going through intentional trauma, to make him or her have this mental condition. That is how people can get others to do things for them that they normally would not do in a normal state of mind.
I actually have heard of the expression of 'breaking the mind' in order to piece it back together, and sadly, many have had their minds broken by adults whom either they trusted or could not get away from. They redo the mind with programming in whatever subject they want to put into the victim.
I am not saying though, that everyone who seems to not have a conscience has been traumatized on purpose as a child. Some of those are people who have suppressed God and removed Him from their thoughts.
A word of caution though; Do not assume that every person who is a homosexual is struggling with homosexuality because he or she has suppressed God in their hearts. Yes, that could be the case. But it might be due to sexual abuse as a young child or some other wrong believe system that they have bought into.
Here is another interesting thought: Most people (usually men) who have raped little girls or even their own daughters, usually will never remember the incident(s). Even more puzzling than that, most of these men seem like very normal people, who live normal lives. But there is a side to them that you may not ever see unless you are the daughter. That is why it is hard for a girl to report a rape. When the rapist is exposed, everyone thinks he is a good guy, because, in most of his life (which is visible to the public), he seems like a normal person who wouldn't do anything like that.
I heard a sad report yesterday, but it ties in with what I am saying. As I watched the news, the reporter announced two different incidences where Bill Cosby was said to have raped two different women. I don't know if this is true or not, but, the point is, even though we have good talents and even wisdom, we can still fall into sin. Even Bill Cosby can sin, and he may really be guilty of what these women are accusing him of. One reporter asked the woman why she didn't report it when it happened. She simply said, no one would have believed her.
* For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who by their unrighteousness suppress the truth. For what can be known about God is plain to them, because God has shown it to them. For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse. For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened. Claiming to be wise, they became fools, and exchanged the glory of the immortal God for images resembling mortal man and birds and animals and creeping things.
Therefore God gave them up in the lusts of their hearts to impurity, to the dishonoring of their bodies among themselves, because they exchanged the truth about God for a lie and worshiped and served the creature rather than the Creator, who is blessed forever! Amen.
For this reason God gave them up to dishonorable passions. For their women exchanged natural relations for those that are contrary to nature; and the men likewise gave up natural relations with women and were consumed with passion for one another, men committing shameless acts with men and receiving in themselves the due penalty for their error.
And since they did not see fit to acknowledge God, God gave them up to a debased mind to do what ought not to be done. They were filled with all manner of unrighteousness, evil, covetousness, malice. They are full of envy, murder, strife, deceit, maliciousness. They are gossips, slanderers, haters of God, insolent, haughty, boastful, inventors of evil, disobedient to parents, foolish, faithless, heartless, ruthless. Though they know God's righteous decree that those who practice such things deserve to die, they not only do them but give approval to those who practice them. Romans 1:18-32 ESV
