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

A belief in a god is not the problem.

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

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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THE BIBLE IS THE PROBLEM!

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

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.

Would you let your son wear leggings to school?

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.

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

If I only have a fire extinguisher to defend myself against some threat from people, should I spray them for max damage or just hit them with the fire extinguishers?

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

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

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

Why is there so much evil in the world?

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