Famous actors = more sales = more money to spend on other things
It's not important to the core fans who are on here but is a selling point to a lot of others who are sadly in the majority.
I've never noticed Bethesda advertising their game by saying what famous voice actors they got. Have you? Please show me if so, I might have missed it.
My "casual" non-forum TES-friends never noticed any voice actors being famous in Oblivion. I didn't either until someone mentioned it.
I doubt someone would go and say "OMG! That game got Patrick Stewart as a voice actor! We gotta buy that game!!! ZOOOMG!".
So my point still stands. Famous voice actor = big waste of money that is much much better spent elsewhere.