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

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

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

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

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

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

A belief in a god is not the problem.

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THE BIBLE IS THE PROBLEM!

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

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.

To a flat Earther, what's wrong with the idea that gravity is simply a force inherent to space which operates only in one dimension? Why do they go further and try to deny gravity rather than just saying it's different than physicists claim?

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

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

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?

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