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

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 .

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

What do you say after "Hi" when chatting?

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

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

Why do I feel worthless most of the time?

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

THE BIBLE IS THE PROBLEM!

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

Why is Reagan seen as the best president in the USA when he literally destroyed the American economy with trickle down system and was strongly against worker unions?

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.

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?

Is it necessary for people to wear towels while showering at gyms? If so, what are some ways to prevent the towel from slipping off and exposing oneself?

A belief in a god is not the problem.