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

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

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

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

Is it possible to achieve spiritual enlightenment while being in a romantic relationship?

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

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

A belief in a god is not the problem.

Jaire Alexander’s Contract Impeded Packers’ Trade Attempts - NFL Rumors - ProFootballRumors.com

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.

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

THE BIBLE IS THE PROBLEM!

Box Office: ‘Ballerina’ Arms Itself With $10.6 Million Opening Day, ‘Lilo & Stitch’ Gets Third Weekend on Top - Variety

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

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.

BYD sells 382,476 NEVs in May, overseas sales hit new high - CnEVPost

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?