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

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

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

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

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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!

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

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

A belief in a god is not the problem.

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

Do you regret being married to your current wife?

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?

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

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

Why did Cartman love Heidi purely with heart, her being the first one he ever did, but then one day Butters tells him that all women are manipulative and then he began to believe that she was a bad person and pretended to be a victim?

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.