Because most people don't go out of their way to CHEAT on unlocking their achievements?
Just a guess.
Point went over your head.
It just wasn't a very good one.
It doesn't matter that "you can". Most people don't. Most people aren't even aware this is a possibility.
I've heard the random dude like you coming out of the woods from time to time telling everyone you can cheat Steam achievements for YEARS at this point and guess what? I cared so little that I still don't know exactly how it works, because I didn't bother going looking for it.
On top of that achievements are inherently "worthless" and they mean something exclusively to the extent someone pays attention to them.
Incidentally I'm on all three ecosystems (Steam, PS4, a largely unutilised Game pass) and guess what? Steam achievements are the only ones that I occasionally check (both mine for the sake of completionism and my friends' ones just to compare what they played) and so that have some arbitrary, fictional "value" to me.
