Well, without some sort of limitation in place: Move to the enemy group, Fireball, Action Surge, Fireball, Misty Step away...
You certainly can do that with 2 levels of fighter but you've burned 3 spell slots on one encounter. In addition, you'd have to wait till 7th level at the earliest when other full casters were tossing fireballs at level 5. In PnP where long rests matter, that's a significant loss of resources. But in BG3 currently, why not? Just rest up and repeat.
Yeah, that's the main problem currently. The rest system in its current state means that every single spell/ability is practically per-combat, if needed. But there is now a whole thread regarding the rest system, so let's not discuss it here.
