BEcause it's a well known fact for years now that blank slate characters and voice acting don't mix.
In theory they do, just in practice it's impossible. I'm sure that even you would enjoy voice acted character more if you had an option of voice with the right personality for every character you came up with, and voice acting would not reduce the number of available dialogue options.
That would just be a 500+gb game with insane development costs.
I think that our main disagreement here with people that like voiced PC is that we like to play unconventional characters while they like to play "Tav" so it's much easier to satisfy them with a voice.
