Wizards hang around reading and studying, but with their low base skills, they can't be very good at many knowledge skills. To be any kind of a successful wizard, they have to keep Spellcraft max'd and to be any kind of successful adventurer, they have to keep Concentration max'd. Most wizards are going to keep Knowledge: Arcana up there as well, so for a wizard with a 16 Int, that leaves 2 skill points per level to get any access to the knowledge in the books they read.