Take a look at the current generation of kids outside (13 14 etc).
Theirs the answer. (At least in UK, our streets are like a battle field LOL)
People simply don't bring up their kids how they used to, they don't teach them what's bad and good properly (I used to get a hit if I was naughty.. I soon learned LOL), nor do they bring their kids up with manners.
And it's also that a lot of people simply forget they ever posted. Or otherwise are just plain ignorant.
And ones who do post thanks. Good on them!
As for me.. Do I post thanks? Yes Always if it helped me.