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

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

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

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

Largest map of the universe announced revealing 800,000 galaxies, challenging early cosmos theories - Phys.org

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?

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

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

Common herbal supplement used to beat stress linked to liver toxicity - Times of India

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.

THE BIBLE IS THE PROBLEM!

A Biologist Spotlights The World’s Most Aggressive Bird. Hint: It’s Flightless, It’s Not The Ostrich, And It’s Been Known To Kill People - Forbes

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

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

A belief in a god is not the problem.

Have you ever seen a woman having sex with a dog?

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