Magic is comparatively rare in Hyboria. It is a difficult matter for sorcerers to draw on supernatural forces. Hence spells of any kind are not common. The reason for this is simple -- the game is based on the works of R.E. Howard. In his 'Conan' stories, magic was not common, and sorcerers did not walk around zapping people with magic missiles. The game's magic system reflects this fact -- as well as the fact that most sorcerers were rather vile individuals (sacrificing humans for more power, taking black lotus, etc.).
(The scholar uses a d6 for HD.)