I see classes as a matter of training. PC type classes are the good training that costs more money while the NPC classes are the poorer training that doesn't cost as much or require as much time or dedication. Thus the rich, powerful or simply hardcore get the PC classes while the poorer and less motivated people get the NPC classes. The noble's son who gets formal training in weapon and armor use ends up being a fighter while the farmer who just likes to fight and may train every now and then with the militia ends up being a warrior. Similarly, clerics are the product of training by other clerics, perhaps with a long tradtion while adepts are simply people who are devout and holy enough to hear the call of their god. Thieves, ....err, I mean rogues, are an odd point as not all PCs present themselves as being trained by organized thieves guilds or even another theif. These types are simply the hardcore and strong willed who really like what they do or find it come to them naturally. Same goes for sorcerors. Someone less dedicated to the lifestyle might simply end up an expert with some similar skills.
Personally, while the NPC type classes are the majority, I don't have any partiucular shortage of PC clases in my NPCs. Probably more than is presented in the DMG under the community generation system, at least for large cities and such.