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

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

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

THE BIBLE IS THE PROBLEM!

I found out I have cancer—I have not told my family. We can’t afford the treatment anyway. Should I just say nothing and let nature take its course?

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

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

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

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

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

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

Is there a possibility that we are living in a simulation and that there is a concept of rebirth?

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?

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

A belief in a god is not the problem.

Who is the dumbest law enforcement officer you have ever encountered?

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.