Because to every individual player their charcter is their very own creation. There would need to be hundreds of different voices to choose from to make people happy rather than just giving them one default voice for their character.
One person may play a female Breton as a hard [censored] rebel another may play the same race and gender as a flirtatious theif. Both would need different voices to suit their personalities and there are dozens more apart from those 2 examples.
Simply if you gave every characer 10 voices to choose from thats 20 per race (male and female) that's 200 fully voiced characters you would need to record which would 1) be a waste of their budget, 2) take up a ridiculous amount of memory space and 3) even with 200 voices you still couldn't please everyone
It leaves more to the imagination. If there character spoke then that's how they would sound. If they don't speak, they can sound whatever way your mind can make up.
What I was trying to say but much more snappy^ :goodjob: