Not necessarily. Philosophers are pretty rare, but in my life I've met dozens, and have more than one as a friend.Which makes a campaign really, REALLLY weird when we meet more than the one wizard that's in our group. If wizards are that limited (and note, I still have no idea why we're focused on wizards since clerics and druids would have just as huge of an impact on a setting) then how on earth have we met three of them by the time we're tenth level?
You can't have it both ways. You can't have "wizards and casters are so super duper rare that no one would see one" and "wizards and casters are so common that we bump into several every single adventure".
Supervillains are pretty rare, but Superman and the X-Men meet heaps and heaps of them.
What I do think tend to run your way is that a lot of D&D material treats magic-users and clerics as essentially ordinary parts of the social fabric: in Gygax's rules, for instance, all large groups of NPCs tend to have at least a couple hanging out with them, they are indentured into city guards and watches, etc. That part of his game isn't REH-ish at all. 4e D&D seems to me to have a similar vibe. I don't know 5e stuff well enough to comment on it.