Because they're loving stupid and want to ruin everything they every had behind themselves ever since they were on the forum. Not only that, but they always come back and
there are legit people who left the community who never said a word.
Honestly it's a waste of money to make a leaving post and come back.
However I will be okay when people are explaining to others why they'll gone or be taking a break for a while, but since it still technically counts as leaving just do it silently please.