That is a good question, but the answer is a bit more difficult, and it depends on if you are asking for an in universe explanation or not.I don’t really understand a thing, and I’d like to.
In every game that isn’t specifically about doing magic, folks expect to be able to play a wholly non-magical character. I’m building a game of my own, and I am having trouble seeing reasons that anyone who has magic as an option would choose not to use it?
This relates to the non-magical Ranger thread, but it’s more about the thematic notion of fully mundane heroes in a world with fairly common magic.
In my game’s setting, anyone who is exposed to magic and chooses to practice and study it can learn magic. This means all PCs have magic skills available to them, and all archetypes have magic skills on their skill list, though some only have 1 or 2.
I guess the question is; why would someone choose to be a hero/adventurer/etc and not want to learn any magic?
In universe it is entirely possible that everyone would learn some magic, assuming the setting works like that. But there is a lot of variables there. Does magic take significant effort to learn? That means there is an opportunity cost, time where you are not learning something else. Depending on what magic can and cannot accomplish, that could be a reason. Incidentally, that is one reason I have a bit of a dislike for Gishes and stuff like Tenser's Transformation, because if magic can replicate fighting skill, then yes, it makes little sense to train as a non-magical warrior. Same with other skills(Why be a sneaky rogue if you could turn invisible).
Another in universe explanation is that having non-magical options could be useful in the case of certain situations. Putting all your eggs in one basket means could leave you vulnerable to an antimagic field or dispel magic effect. Having some non-magical specialists could be useful in such a situation.
Ultimately though, I would agree that looking at it from an in universe point of view, not learning any magic in a world where it is trivial to do is like not learning how to use a computer in our world. You might be able to do so, but it's not really practical.
Which brings us to why a player might not want a character with magic, and that is a pretty easy answer. D&D allows us to play out the fantasy stories that inspired us, and for every player who dreams of playing Merlin, there is someone who wants to play Conan. The stories that make people want to play D&D are full of knights, swashbucklers, warriors and scoundrels who don't use magic even if it exists in their worlds. D&D wants to allow all these character types, your mileage may vary on how well it pulls it off I suppose.