Though not done by every user, I really think that the reason new users make bad threads/posts, is because we influence them.
Every new user makes bad posts when they first start on any forum, that is understandable. Most users learn the way of a site, and their posts gain in quality and eventually they become a "regular".
There are however, some users who have had some experience where a lot of new users totally make fun of their posting quality in a thread they made.
Though it may be fun to make jokes at a new users expense, in the new user's mind, they are emotionally upset in some way. These emotions will either cause them to strike back at the "regulars", stop posting all together, or make terrible threads.
The latter result is something a lot of users are complaining about lately, as if they don't realize what their actions are causing.
Since this is really the most recent case, I'm going to use Sonicflare9 as an example.
He is not exactly a new user (he signed up before me) but he is new to the forums.
He makes some threads about trying to get girls, and people make fun of him for because of that. Yes, his posts are bad, but do you really think harrassing him is really going to make him a better poster?
My solution to this problem needs support on both ends.
The "regulars" need to lay off new users who's posts are lacking in quality, and the new user's need to try and change by learning what is acceptable on this forum.
Please try and welcome new users, not isolate them. Who knows, we might just get another Asandir or POTaTOS out of it.