Wj
Why does the wizard have to cast a low level spell? Either they go first and cast a level appropriate spell that kills or incapacitates enemies, or they go after the bad guys and burn a 1st level spell to possibly live long enough to cast the spell they were going to anyway.
What has shield done that is so overpowered , let the wizard live an extra turn?
It's a
reaction.
It's not competing with other spells.
You can cast that other, level-appropriate spell with your regular action, incapacitate one opponent and then make yourself virtually unhittable by anything still standing for the rest of the round by casting shield (especially at higher levels, when low-level spells you can cast with your action aren't that useful anymore).
Or you can spam it on top of damaging spells when dealing with something you can't incapacitate (such as monsters with LR).
I don't really get why you seem to be comparing shield to high level spells, it can be cast on top of those (and is functionally a stun for anything that wastes your turn missing you).
It's a first level spell that doesn't require much commitment (as it can be cast out-of-turn, and only if something actually attacks you), lasts until the start of your next turn and provides a very significant bonus to AC even at high levels.
What it does that is so overpowered is, potentially, saving the wizard (or really, just about any class and subclass that can cast it... like, just to name a few, Bladesingers with their already insane AC, Valor bards medium armor+ shield, Eldritch Knights in plate armor+shield) a huge amount of damage for a trivial cost (a 1st level spell).
And yeah, in a game where combat is based on attrition, "fight (rather than "live") an extra turn" is kind of a big deal.