As I explained to
@HammerMan we have four spellcasting classes that all cast arcane magic and the primary lore difference is where their magic comes from. Origin, power source, narrative. If you take that away, and opt for a generic "not explained" answer, the primary narrative difference in these classes goes away. Narrative matters. Origin matters.
You could make a generic arcane caster (let's be old school and call it Magic-User) that gets spell slots and little else. It can represent bards, sorcerers, wizards, warlocks, psions, witches, or mysterious users of magic. The DM and player can decide. We can do that and condense magic into one single class with no history or origin, just a bunch of supernatural abilities. But it would be hollow, lifeless, a blank slate.
I don't want that. For magic users or for fighters.