I'm glad you bought BG3 and all, but if you hate D&D, why did you buy a D&D game? I mean, why are you playing a D&D game and then upset that they made it based on D&D rules?

Support and mages classes do magic, but they also need to swing weapons and shoot with crossbows. They differ from archer or warrior in the gameplay because they aren't as good at these things AND their magic makes them more potent when handling bigger enemies. As you gain levels and become more powerful, they become VERY powerful especially as AOE and Damage Mongers. They need to be limited so that they aren't overpowering the rest of the classes.