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

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?

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

How to take pictures of the Northern Lights with Google Pixel and other Android phones - 9to5Google

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

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

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

What is something brutally honest that needs to be said?

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

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?

What makes outside showers appealing? Why are they not commonly seen?

THE BIBLE IS THE PROBLEM!

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

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.

Do you think Lady Gaga and Celine Dion have rehearsed separately for their performance at the Olympics opening ceremony?

A belief in a god is not the problem.