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

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

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

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

Why does my vagina smell sort of fishy/musty days after sex when my boyfriend ejaculates in me? There isn’t any itching or burning when urinating, so I don't think I have BV. It just doesn't smell like me.

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

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

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

What do you do you do if your motorcycle chain snaps while riding on the highway?

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.

A belief in a god is not the problem.

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?

What are the reasons for your political affiliation with the Democratic party? What are some aspects of the party that you support and some that you do not?

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

THE BIBLE IS THE PROBLEM!

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

What are some reasons why people may fear strong men?

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