I mean divine magic is 'just spells'. But we have a separate cleric class. Nature magic is 'just spells' but we have a separate druid class.
Psionics has an even bigger excuse to be different as it's not historically used the vancian casting spell slot system, but for some reason it's now just shoved into subclasses.
Even the sorcerer gets its own class, and it's a discount wizard with a feat glued to it.
If it's all 'just spells' why not axe the warlock, sorcerer, druid, bard, and cleric. And make a single 'magic user' class. It's all just the same magic right?