Why does high fantasy seem to be skewed towards telling the reader all about the protagonist's childhood and training even when the exact details have minimal bearing on the plot of the book or series at hand?
I have a number of mutually contradictory theories about this, and of course they may all be wrong; please feel free to poke holes with wild abandon.
1) High fantasy readers are more focused on setting than the readers of other subgenres (at least while they're reading high fantasy). Therefore long passages that don't advance plot much but give plenty of opportunity for setting to be expounded upon are a virtue.
2) High fantasy readers are more focused on character than the readers of other subgenres. Therefore the details of how someone became who they are become more interesting, even if they're not doing much of what they do yet.
3) High fantasy readers have more difficulty than the readers of other subgenres with picking up on details of character or setting and want them exposited much more explicitly and slowly.
4) High fantasy readers are looking for books of substantial size, because they give more room for a leisurely pace and side paths of whatever kind, and this is one of the common side paths taken.
5) Many writers would love to tell their readers about the finer details of their characters' childhoods, but bookstores are not as keen on selling other subgenres at the same length, so their lovingly detailed prose is ruthlessly slashed.
6) ??? (your turn)
Whatever the explanation, I have some issues with the structural/thematic constraints this ends up imposing. If the discourse on the hero's childhood is not to be completely irrelevant, similar issues must recur in adulthood; very few people write at length about how our hero conquered a fear of heights, only to make tall buildings, cliffs, flight, etc. and the former fear of same completely irrelevant to the rest of the book. Where this really starts to bother me is in their relationships with other characters: either the hero meets the nemesis at the age of 12, or the nemesis bears striking similarities to the childhood version of same. And you know what? No. Most of us don't marry someone we knew when we were twelve (my parents notwithstanding), and while many of us can spot recurring issues in our lives, we sometimes do actually manage to move past them! Into new, different, ickier problems! Tell me: your arch-nemesis in junior high. How relevant are they to your life today? How directly, literally relevant? When was the last time you saw them? Did you still care? The It All Began When I Was An Infant school of high fantasy writing is alarming to me in that sense: it didn't all begin when I was an infant. And I don't think it has to be that way for characters, either.
Novels where something interesting and plotty happened in the protag's childhood are not at all what I mean here.