Literally anyone can take the Initiate feats.No, these can understand only SOME part of magic, but they can't do anything about other parts and specifically not cast a spell. And some will never be able to, since multiclassing is limited at least by stats. Adventurers are not common folk, and even them have limitations in that domain.
Bards get a class feature to steal spells from every other class.
Nothing in the game keeps any rando farm boy from becoming an adventurerer.
Again, there are entire species literally born with magic powers.
While a lot and I would say most people ignore it, magic is a part of life in D&D Land and there's no way people haven't studied it.
Yes, any of it can be changed to taste, but what's there indicates a way more magical and fantastic world than people actually run it as. I mean IIRC, at least 3 entire people can just alter self at will -- how is the world not affected by pretenders and effortless beauty all over the place?