No reason at all, except the random nature of the merchant's stock makes it difficult - and very frustrating to pick up the low- and mid-level skills that are the basics of a character build.
Effectively, this game mechanic currently makes it quite unlikely that a given character will be able to develop into the one the player wants to play.
This irks a lot of people, me included.
Excessively random generation of epic spell/skill books only as loot would likely put the same damper on the endgame.
If that's the kind of game Larian wants to produce, OK. It's their game. But it will be a game that disappoints a lot of players, and therefore not a long-lived or favorite game for those players.
Basic marketing.
