THP is usually harsh because someone hates your guts (rare case) or because they find that they can only tell you how to write better through harsh criticism.
I'd like to point out an analogue in Dieter Bohlen from Germany's "America's got talent". He finds the harshest words for people that can't sing, destroying their dreams almost instantly, but for a good cause. The music industry, much like writing, is a very hard business to succeed in if the right people on top don't find you good enough.
Dieter said that he might have had technical talent, but his voice sounds awful unless he's falsetto. He was lucky to find someone pretty, Thomas Anders, who could only perform five tones well. Together they were two shitty singers, backed by catchy music, but at least they knew what they were good at: one at falsetto, the other at five tones in mid-range. Together they were Modern Talking, the most popular German act since the Nazi party.
They were only successful because they knew themselves well and got lucky. Those young people with mediocre talent and nothing else to stand out from the rest, though, they won't make the break.
tl;dr - harsh criticism usually hides good intentions, but that's the good impoliteness, most are already aware of that.
Second, the problem with writers not coming out to write is mostly performance anxiety. I know I have it, I can't write a single thing without fearing that it'll be a bunch of crap. Whenever I overcome that fear, I actually wrote, but then I lacked the persistence to keep going, for one reason or another.
Once I wrote myself into an imaginary dead end because I didn't know to write and pace a freaking birthday party. Next I got bombarded by anon in a fresh story, one persisted that my character was a Mary Sue, I couldn't defend it with anything, he'd just tear my arguments a new ass and turn them against me.
The wish to write remains. I've got ten story concepts in different stages of planning, some are just beginnings and endings without a proper middle, others are reconstructions of older stories that range from 150 to 1200 pages, one is a proper high school year dissected into sections with all possible ideas surrounding events. I made the latter for a possible Neo-GH, not that I have the guts to try that kind of stunt.
tl;dr - performance anxiety and sensitivity to criticism