What Is The Main Reason Atheist's Claim There Is No God?

There is absolutely no evidence to suggest that such a being exists, or ever existed. Rational people therefore tend to be atheists, or at least agnostic.

Generally atheists claim that there is no God because they have come to the conclusion through some logic that there is no God.

Because something has happened in their life where they called upon Him and it didn't work out the way they wanted so they got mad at HIM.

The main reason that I am an atheist is that there is absolutely no reason to believe in any kind of deity at all.

When you can explain to me why you don't believe in Zeus, you will understand why I don't believe in your god.

Religion was originally created by people who were unable to explain the natural phenomena going on around them. To them, gods must have been responsible, since they lacked the tools to discover the real explanation. The Aztecs sacrificed multitudes of their own people to try to ensure there was rain, and enough food, and a favourable outcome in battle with others. They didn't know what "climate" is, or fertilizer, or how to vastly improve their weapons.

"God did it" is an easy answer to everything - there it is, all laid out and prepackaged for you. No thinking required to understand your world or the people in it or the events that take place.

Atheists are people who are not satisfied with that answer. We want to study reality, and learn, and grow in our understanding of our lives and our world.

And I have to add, To Intuitive:

All the analogies that you draw when trying to argue for a "creator" fail because you are talking about inanimate, static, objects. Once made, a watch does not grow, learn, or adapt to a new environment. It can't. It just is, and it just does what it does.

As for accepting that there are some things that we have to rely on experts to tell us, that is equally invalid. Sure, I might never have seen a dolphin give birth and nurse its young, but I could if I wanted to. Same with measuring the distance to Honolulu - if I wanted to, I could go down there, rent a boat, and measure the distance. We rely on facts that have been verified by experience and observation, and that could be replicated if we wanted to put in the effort.

There is no need for us to disprove a god, because there is no evidence that one exists in the first place. It is the one who is making the claim that must prove it. If I said I had a car in my garage that goes 800 mph .... Do you think it's up to me to prove it, or up to you to disprove it?

To say that there is no God is just as much a claim to knowledge as to say that God does not exist, so both views require justification. It's not enough to simply show that all the arguments for the existence of God fail, but the objector must give justification to the contrary, otherwise the said objector is really just a soft agnostic--the possibility is still there. For example, in ancient times the Romans lacked a belief in the existence of Native Americans, so does that therefore mean that there was no Native Americans? No, it doesn't. The possibility is there, but it just wasn't a justifiable belief prior to any transatlantic explorers.

Because they don't want to accept a power over themselves whom has a set of moral standards by which they should live.  This allows them to do whatever the hell they want without feeling any guilt.  At least they would like to think that they won't feel any guilt by disbelieving.