It's not about having a different person speak the line, but having multiple people speak the same line. If I have 5 different voices that each do a "Hello"-type greeting, that is only 5 audio files they need for that. If there are 100 people in Burma, roughly 20 people would have the same voice, and all 100 people saying hello would still only take 5 audio files (with much repetition). If they now had 100 voice actors, each one would require an audio file for the "Hello"-type greeting. I now have 100 audio files, which will take about 50 times the space. Person A and Person B can (and usually did) usethe same audio files in Oblivion.
That still doesn't really make any sense. You're correct that individual people doesn't really get their own greetings, but gender/race do. And tere are 2 genders and 10 races, so there are 20 different "Hello"-type greetings.
And then there's the other point where the 360 disc could hold up to 6.8GB back then (it became 7.8GB not long ago) and Oblivion is about 2GB less than that. Even with uncompressed audio it would take quite some time to fill 2GB with greetings.