Why does it matter so much to atheists that God doesn't exist?

The people of the modern world have evolved past all of that nonsense.

IT does not matter, mainly because it does not exist.

Because these are not any gods rules we are talking about.

I’ve Figured Out a Method for Keeping My Son’s Favorite Toy Clean. It Might Lead to Some Trust Issues. - Slate Magazine

Notice that I said their rules, not any gods rules.

Those rules that you are trying to reinstate are rules made by men who didn't know anything about the earth, nor anything about all of the other inhabitants of it. How can you not see that?

A belief in a god is not the problem.

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I am speaking to the US Bible thumpers here, of course ~ many of whom are flat-earthers who deny evolution and see scientists as evil people who are constantly trying, and succeeding, in disproving their beliefs one by one.

These are man made rules - made thousands of years ago for the use of the humans who lived thousands of years ago.

And then use and ENJOY these fantastic scientific advancements every single day of your life?

Scientists Successfully Simulate Bizarre Phenomenon Where "Light Appears to be Generated from Darkness - The Debrief

I do not personally care if you believe in a god or not.

And how do you deny hundreds of years of advancement in all fields of science?

THE BIBLE IS THE PROBLEM!

Was Jesus Christ Jewish?

My problem is that people who try to shove their rules down everyone's throats - including by physical force - DO EXIST .