Why do they lose their accents?

It's been a while since I posted (Christmas holidays and all!), but this question peaked my interest enough to discuss it here.

http://www.eonline.com/gossip/answer/?uuid=4538d873-bdd5-4823-973e-d3606e9bf036

Why do foreign actors try to lose their accents? Do they think they won't get jobs if people know they are from a foreign country? There also seem to be an awful lot of Aussies who do their best to lose their accent. Portia de Rossi, Anthony LaPaglia, and the like. I have seen Australian movies where both of them use their natural accents and they sound fine to me. Both can admirably pull off American accents as well. Why can't they just talk with the voice they grew up with when they are doing interviews? I think we can all understand Aussie accents, can't we?