So, here's a thought. NPC's are defined entities. They have substance, character, personality. Or at the very least the potential to, always. Their will is confined to their own existence, to the extent that they exist.
Playable characters, on the other hand, while occasionally possessing these things, tend more often to be empty shells. It is not uncommon for them to have no voice. They are skin suits puppeted around by some abstract will that is entirely foreign to their being. Their victories are not due to their own triumphs. Their failings are not due to their own faults. Their feelings towards shorts are not due to their own opinions on comfort and ease of wear.
Point being, We would much rather be an entirely scripted entity that consistently stood for itself, than an ever-changing shiny set of wheels that's only purpose was to be taken for a ride by another set of hands.