Because your parents in the country you come from pumped it in your brains and this is because your country told them too..
Such might be a start for a person who believes in God. But if he continue to focus on it after he has grown grown to adult age, he is actually, mentally a child.
What people should be doing is starting to look at the technology in nature. And they are. They are trying to duplicate some of it. Many people of science ARE duplicating a little of it. And you can ask them if it is easy. They will tell you about how difficult it is, and that if they didn't have the help of a lot of other like minded people, and financing from a bunch of financiers, they would have failed in their attempts to duplicate a little of nature, long ago.
Where did the technology of nature come from? If it can't be duplicated easily with a lot of thought, why would anybody think that nature could make itself, without any thinking at all?
There is only one answer... somebody who has the ability to think a lot better than people, and has a lot more power than people - even hydrogen bomb power is tiny when considering all the power locked into the material of the earth - and has a lot more ability than people, to be able to put this whole thing together.
Why would anybody rather remain with the child-like understanding that God DOESN'T exist, because that's all their parents taught them? They have grown up. They have a chance to be an adult, and think on their own, and calculate for themselves, that nature is a mindbogglingly great piece of machinery, greater than can be found anywhere.
Machines have makers.