Adventurers aren't common in the world, though the classes are represented well enough by NPCs and PCs (former and current). Magic users in particular tend to be rare, with a metropolis of 100,000 able to field maybe 30 or 40 individuals with the talent and drive for any given class. Obviously many people can rise up to the challenge of becoming a low hit die NPC from the back of the Monster Manual, but for anyone with 10 or more hit dice/levels, they are among this exceptional few. Schools exist to train nearly every type of adventurer, though they often extract a tithe from successful adventures to fund themselves.