The problem is that there aren't really any good ones. Oblivion Character Overhaul is decent, but far from great, and it doesn't fix the issue with the elves looking like pointy-eared humans (which they shouldn't). And I haven't seen a single mod with professionally re-recoded dialogues.
I am mostly in agreement with you. That's why I said potentially. The modding community probably doesn't have the resources to hire professional voice actors, but I don't see why someone hasn't done it themselves. I don't really have a problem with the voices in Oblivion to begin with, but there is a lot of recycled dialog that gets noticeable.
I mostly used the overhaul mod, some basic gameplay ones, misc atmosphere/weather mods, and some others when I played. I always tried to stay away from the mods that added new quests though, because the lack or voice acting in all of them kind of ruins immersion for me since everyone else in the game has spoken dialog. I like consistency I guess.