I have a suspicion that most prospective writers don't have a clue about what writing is.
edblog (who sits next to me at work) used to be a slush-pile reader, and he tells me that most of what passed by him was quite illiterate. In the discussion thread I linked to above, someone asks 'Are the manuscripts submitted to publishers generally of lower quality than, say, the average story on fanfiction.net?' The answer - tragically - appears to be 'yes'. Perhaps writing to novel-length exposes weakness more harshly than fanfic-length. Or perhaps the effort and persistence involved in writing 120,000 words is only likely to be expended by those who know they have a measure of competence or those who don't have a strong idea of what competence is.
I'm not suggesting that if you write a novel and can't get published then you are deluded and incompetent. Some unpublished writers must miss publication by a narrow margin, or for reasons of narrow commercialism, or because their skills have not quite developed far enough. However, these near-miss writers are in my opinion a minority and the miss-by-miles category is vast.
It is possible to turn near-miss into hit with practice and effort. For the other type of writer, this improvement is not a possibility. That's what makes the subject so tragic.