The rule implies a specific type of setting. The truth is in very traditional and feudal setting, the training required to be a "D&D style adventurer of a PC class" would only come from specific backgrounds. And in classic settings, only certain types of those people would leave "normal" society and become adventurers.
I always found a certain charm in Basics heavy emphasis on archetypes. It was a game where you WERE an elf, a wizard, a ranger, a thief, etc. It's not the one where you refluff your thief as an honest merchant, you are a sneak thief. It works because the archetypes are strong but have some flexibility. But you gotta be into that strong archetyping and that can get limiting or lead to "a new class for every idea"