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

A belief in a god is not the problem.

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

THE BIBLE IS THE PROBLEM!

Dejected Joshua Kimmich reacts to Germany’s Nations League defeat - Bavarian Football Works

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

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?

The FC Bayern squad for the Club World Cup - FC Bayern Munich

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

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

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

How far back into your childhood can your remember and what is your favorite memory of that time?

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

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.

Why are you a Muslim? Why is it Islam for you and not something else?

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