The same reasons elven boys aren't into girls*: tradition. Clichés are an important part of the world in heroic-fantasy, one of the basic bricks of the universe, may we say.
Besides, old grumps and whimsical powers don't mix easily.
* And thus, all those half-elves from elven mothers, who've gone searching for love and lust in the arms of manly humans. Read your Tolkien, there were at least three occurences of inter-species marriage, and each time it was a human las and an elven lass.