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

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

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

THE BIBLE IS THE PROBLEM!

Boy, 8, dies hours after complaining of headache when infection spread to brain - The US Sun

A belief in a god is not the problem.

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

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

What made you feel disgusted today?

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

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.

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

Why do SpaceX rockets keep exploding? - The Verge

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?

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

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

Greetings from Warsaw, Poland, where the flags are flying ahead of a key election - NPR

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