It's pretty clear that atheism helps its adherents in various ways. If they feel it does then it does. There's no point arguing against that.
As someone who was once an atheist and now am a theist, I'll take a stab at these questions as I find them interesting. Thank you for the thread.
Atheism, at least for me, was a state of not believing in God; I don't ever recall thinking whether my atheism was helping me or not.
However, does atheism help? I'll only speak here in this paragraph to the time when I was an atheist. The answer for me will be both no and yes. To explain my answer, I'll say that when I was an atheist, I relied completely on myself and took personal or professional failures hard. That's because there wasn't any comfort I could derive from an external supernatural force and so to the degree that theists are helped by praying and meditating, I wasn't at that point in time helped by my atheism. However, on the other hand, atheism does help in the sense that a person then is not concerned with what God's Will and a person feels free to exercise choices that are within the purview of his/her moral compass without looking to direction outside of himself/herself.
What I don't understand is why the atheists can't leave it at that?
There are different types of atheists: Some atheists have had negative experiences with theism in their personal life. Some atheists never believed in the first place. Atheism generally, I suppose, comprises of just a collection of individuals that are not unified by anything other than their collective disbelief in God. So, as to why can't maybe specific hardcore atheists leave theists alone as to maybe specific critiques, it is because atheists observe theists acting in a manner that they find not only sometimes strange but other times evil.
Why is it so important for them that God does not exist? Why can't God exist?
When I was an atheist, I don't know whether I would say it was important for me that God didn't exist. I frankly didn't care? And I also felt then (and still do feel) that theists themselves are generally the most prolific advertisement for atheism. And the reason I say this is because of the observable trends of bad character, bad behavior, intolerance, and anti-intellectualism that can be generally encountered in the theist population. The truth is that even if atheists would become open-minded to theism (essentially some form of spiritual agnosticism), I guarantee you that what they hear or see from theists on the Internet (the medium that acts as equalizer of good and bad voices) will generally be enough to turn them off organized religion and in real life the self-righteousness and holier-than-thou and fundamentalism in some theists (even if it's not most theists) will inevitably garner probably the same result.
Having been on both ends of the spectrum, the truth is that I many times see greater willingness in many atheists to offer compassion and stand up for those "otherized" than I see in theists (though perhaps that trend is an effect of liberalism). Regardless, this is perhaps the greatest tragedy and failure of modern theism. In modern practical application, followers of Buddha are not Buddha-like and the followers of Christ are not Christ-like and the followers of Muhammad (pbuh) are not Muhammad-like (pbuh) and the followers of Moses are not Moses-like and the followers of Hanuman are not Hanuman-like. So, the question then becomes why would an atheist be attracted to the serious moral and character failures of theists who themselves are not only
not serious about being compassionate but are unwilling to stand up for those marginalized and otherized and the upsets they internalize are confined to the moments wherein their beliefs are questioned or criticized.
This phenomena is perhaps why many atheists think it is not necessary to have God in one's life to live a moral and ethical life and consequently also continue to eschew religions and even general study of religions. And also, many atheists might have received unsatisfactory answers from theists in many aspects specific to religion, which has (or might have led them) to believe that God can't or doesn't exist.
Why can't the creation of the universe be just whatever God created? Why not God?
I'd reassert that since I was an atheist then, I didn't contemplate the universe in terms of a theological perspective. So, what that essentially meant is that whether the universe was created by a force known as God was not only then not important to me but I also didn't care for religions generally to contemplate on the matter for any length of time.
The honest answer to "why not god" from my former atheist's position in terms of worldview can be summed up as a shrug and honest answer of "I don't know."
Last Thoughts: My best and sincere advice to both atheists and theists generally is to seek knowledge and imbibe wisdom from people they consider greater than themselves and travel out into the world and be the best version of themselves and live out the best lives they're capable of living as we can hopefully all agree at least that life is short.
Peace.
