> Who decided they were hacks instead and why?

"They" decided. Because "hack" sounds new and cool.

I have to assume that "they" were computer hackers whose successes, I suppose, were "good hacks."

It isn't just "life hacks." There are "cooking hacks," "mileage hacks," "writing hacks," "guitar hacks," you name it.

If you're having hard time doing something, there's probably a hack for it.