Because it wasn't about hit points. Or the number of spells. It was about casting light to blind your enemies. It was about hold person that lasted for an hour instead of a single round (save ends). It was about sleep that could end an entire combat against a dozen kobolds or goblins - instantly. It was about fireball and lightning bolt that could expand to fill their space or ricochet off walls. It was a charm spell that could potentially last for days or longer.Seriously, how did TSR wizards get a reputation for being absurdly OP? Or is that more of a WotC D&D thing?
It was because magic was magic, not just something that could be shrugged off on the next turn.