Not everyone wants to spend their time maintaining aircraft - someone has to design the thing in the first place! Also, universities are NOT trade schools and are not intended to be. The apprenticeship system already exists to train people to maintain planes (and be carpenters, electricians, builders etc) - university isn't supposed to duplicate that.
You don't learn to be a design engineer with hand tools - that's why most design engineers don't go through the apprenticeship system and instead opt for a degree. Hands on experience certainly helps however it isn't where most learning takes place.