As several have mentioned, this is a dramatic tool. Some of the stories wouldn't make sense otherwise. As examples - Why would Pollyanna go to live with Aunt Polly if she had two perfectly good parents at home? If Lilo's folks were around, the whole story wouldn't make sense (all the issues of family and belonging). Nemo's mom & dad would go to look for him and thus no Dory. Just my two cents/

Zeus, and Hera and his mortal parents...umm....Amphitreon? (sp?) And I don't remember his mortal mother's name.
because it has both parents and it is a great story. This has been an issue with my kids until recently (9 and 7). It really bothered them that a parent died--esp. in Lion King--it seems a rather violent death. We always have to fast forward that part 