Why do so many movies use 'celebrity' voice overs?

There was a time this didn't happen. Disney especially, didn't use hip celebrity voices to do their animated movies. The last movie that used voice actors and not regular actors seemed to be The Little Mermaid. After that they cast Robin Williams in Aladdin (1992). By the time The Lion King came out almost the whole cast was well known actors.

Of course it doesn't matter to the kids. It doesn't matter to the adults either as long as the voices are good. I wonder how many voice actors went to unemployment because of giant corporations decisions to bank on celebrity voices.

^^ Agreed.

However, Disney's "Frozen" is a good example of using great voices/singers (who are pretty much unknown outside the Musical/Broadway fandom) without trying to get "big names" to rake in the viewers.

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I think it's distracting when they use celebrity voices. Instead of connecting with the new character, I am imagining the actor that's voicing it. I just see Owen Wilson standing behind a microphone.

Celebrity voices have long been used (ex Disney's The Rescuers starred Bob Newhart and one of the Gabor sisters (I forget which).
The trend of celebrity voices, not to mention incessant pop culture references that quickly become dated, can be traced back to Jeffrey Katsenberg's tenure at Disney (and now running Dreamworks since inception).
Sometimes, an actors voice is a draw (Genie wouldn't be the same without Robin Williams) but mostly it's pointless (sure, I like Patrick Warburton, but the man has no range, no other sound... It's tired and distracting).

Yeah I've never quite understood this. These actors must cost so much more, which means there must be some justification for studios to continuously do this.

Occasionally I think the actors talent improves the character, like Robin WIlliams and Genie, but usually it is just distracting or unnecessary to have those big names.