It's an easy way of giving the main character, well. . . character! ;D If one or both parents are absent/dead, that automatically gives the character a sense of maturity/depth.

but i noticed in all major disney movies someone always dies, even if its the evil one :P ♥

Quite agree with fanfly.

haha my dad used to point that out to me when i was little
ironically now he lives 2,000 miles away and i never see him

their based on traditionally fairy tales, and the death or absence of parents is a reoccuring symbol it's one of the 31 one functions from Propp.