Quality control.
They use big name actors for recognition of the voice, as well as experience. In the case of Bethesda, it's generally to cover up dialog that is absolutely bland (name recognition generally overrides crappy dialog).
I posit this question: If the king in Oblivion wasn't Patrick Stewart would you remember him? If Mario didn't have that thick stereotypical italian (pronounced eye-tal-yun) would it seem odd?
The second that you find an awesome voice actor you generally find a good actor. Good actors have developed skills by playing parts. Playing parts is a career. Thus, you generally only find truly awesome voice actors that are also big name voice actors.
While you may not agree with the logic, it's how dev teams spin the situation. Right or wrong, it helps to sell the gaming experience.