I don't think I've heard of a company that sells them prerolled, but that doesn't meant there aren't any folks out there that do it in bulk, that sell to the public, sorta in a cottage industry way.
Why..., CASH!
A very limited customer base, and most of that knows how to roll their own, so there's no money in it from a mass production point of view. Same reason why there are several brands, and just about every color & flavor in 12 gauge, but us 16-bore shooters have to scrounge to find ammo, let alone the right ammo for the game. Small customer base, small reason to mass produce.
LD