Why do atheists say that God doesn't exist?

Based on my study of the history and anthropology of religious cultures and of modern brain science, I am convinced that the brain wiring of us human beings makes it simple and natural (and probably always has done so) for us to imagine and to believe that there are supernatural agents at work out in the world and that they cause various events to happen or read minds, work miracles, etc.

That some people conclude, sooner or later, that such supernatural agents don't exist is a result of (1) some strenuous swimming against the current of our own brain wiring and childhood indoctrination or acculturation and (2) serious education and serious thinking about how the natural universe and human beings are really structured.

By what authority does the Asker decree that the "real difference" between atheists and theists is not what they believe or say, but what "causes" their beliefs or convictions? I'm not saying that the causes of their beliefs or convictions is irrelevant -- the causes are important and fascinating -- but the content of atheists' and theists' beliefes or convictions are just as real and just as important.

The Asker asked, "So why do atheists tell people that God doesn't exist? What exactly does that accomplish?"

In a world where belief in imaginary supernatural beings has motivated believers to (1) commit terrible, violent crimes against other believers and non-believers and (2) press for the dumbing-down or destruction of science education in U.S. public schools (and these problems are getting worse), calling a spade a spade (or saying that the emperor has no clothes) always has positive value.

One can quarrel with the words used, or with the etiquette of the speaker, or with the time, place and circumstances in which an atheist speaks up and says "Of course, all of these gods are imaginary, just as all of the laws, commands, and other communications that are attributed to these gods were originally made up or discovered by people and only later put in the mouths of gods." But that's another issue.

Finally, the Asker suggests (in what seems like a mighty non sequitur) that the real issue is whether "faith" is a virtue.

To me, "faith" is not hope, or confidence, or courageous, open trust that we can make sense out of the universe. No, the word "faith" has been permanently sullied by years and years of incessant god-talk by the "faithful." If one analyzes how the word "faith" is used by the religious, who definitely believe that "faith" is a virtue, it is apparent that these days, "faith" is most accurately defined as a persistence and certainty of belief that is impervious to evidence and reason.

If you'll accept the above defintion of "faith" as accurate for purposes of discussion, then I hope you'll understand that to me, faith is NEVER a virtue.

If what you're really trying to ask is "why is a person a theist?", then I can tell you...the answer varies.Some were raised into a particular faith and others merely 'tow the line' when it comes to belief in a deity. However, on the whole, I'd say most people believe in 'God' because they have been party to some event or experience that he or she has interpreted as only being possible or true due to the existence of 'God'. Faith naturally follows. The atheist is simply the opposite. The long and short of it is simple: the theist, from his/her perspective, has a reason to believe. The atheist doesn't. It's not about if faith is a virtue or not, but whether there's truly valid reason for it.

No such deity exists. That's the point of atheism.

I have not denied that many religions are "based on God", I just contend this "God" to be imaginary. Kinda like that imaginary friend you may have had at age 5.

OK. I'll rephrase. While this deity is imaginary, Christians base their religion on an imagniary character. Kinda like if somebody based a religion around Harry Potter. Same comcept.

Not all atheists say God doesn't exist. They simply lack belief in a higher power.

"Even according to atheism, belief in God is not the real difference between atheists and theists."

I'm sorry, but you added a "not" in this sentence where it wasn't supposed to be. I had hoped it was an accident, but it seems it was not. You are sorely misinformed and ranting about something which you know nothing about. It's really not even worth it to correct you, because you are so far off base.

The real problem is this.

When you start with a false premise, it often has disastrous results. Especially, when the net total of people who start with this false premise numbers into the billions.

So why do theists tell people that God does exist?

What exactly does that accomplish?

If God exists, why did he make atheists?

God doesn't exist.

Who says it has to accomplish anything? Maybe they are just expressing their opinion.