Griffith Dragonlake said:
All very interesting ideas but . . . they are in conflict with the RAW!
Mmm, yeah. I think we can agree that the RAW refers to just PC's, not to the general state of all the people in the world.
OT
There's several ways of looking at it.
I think it's more helpful to think of magic as an art rather than a science. The vast majority of people have no artistic talent, and therefore cannot be artists or musicians even though they might want to. They can try and try, but at best they might be able to play a couple songs by rote rather than through true understanding of musical theory.
Certainly PC's can multiclass to Wizard whenever the GM lets them, but that's more of a metagaming shortcut than any indication that a person in the 'real' fantasy world could actually be able to do that.
There's also every indication that learning magic can be dangerous to your health - mess up a sonnet and you get booed off stage. Mess up a conjuration or evocation and you can easily die. That alone will keep most people from learning it.
Also, look to your fantasy novels for reasons why people would not do it. Wizards tend not to share their knowledge - they spent years learning it and they're not going to waste that hard-won knowledge on a bunch of farmers or by being shopkeepers. Such things are for lesser men.
There's a major chink in all of those explanations and it's called
The Bard. The Bard doesn't study magic, he literally just picks up bits and threads of lore along the way and he's able to use them. He can even technically cross the arcane/divine divide that way.
There are several novels where people do, in fact, learn a bit of magic here and there. Most people might, in fact. Really, the idea that they would not is an artifact of D&D's class-based system where Joe Blow can't just spend a couple skill points and learn Light, or Mending, or Cure Minor Wounds.
It really depends on the tone you want in your fantasy world, the nature of magic itself, and several other factors.