Especialy when Bethesda has habit to waste money on some 1-2 celibirities,
Voice acting does indeed take disk space, but so do great graphics. Atmosphere
is depth, after all. It's part of the unspoken qualities of the game. The game isn't just
a game, after all, it's also an
experience, Gameplay is crucial, but it isn't the only vital aspect of the game. If it were, we could make do with sixteen-color graphics, low-polygon-count models and MIDI music, with text dialogue. The gameplay could still be excellent...but you would have lost all of the visual wonder and atmosphere of the game. You can dismiss all of that as unimportant, "surface", a "shiny wrapper" or whatever, but it isn't just extraneous bells and whistles. Having a real solid sense of design in the visuals, the voice acting, and the music is just flatly essential.
If you want to play a game - a good game overall, actually - that obviously saved money on their voice acting, go play Sins of a Solar Empire. A fairly popular RTS, I believe, with a lot of scope, but the voice acting is so bad that I actually get a little embarrassed if somebody is in the room and hears the speech in the game. It's that bad. Wat you call "celebrity voice acting" can also be called "paying for quality voice acting." It does make a difference.