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

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

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!

If gays can get married, why can't I marry my dog or a cheeseburger?

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

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.

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

Can you list every album you have ever listened to?

A belief in a god is not the problem.

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

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

Why do flat Earthers still exist even though it is scientifically proven that the Earth is spherical?

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

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

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?

Why do people stay in cults after they have joined?

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