I play a wizard (currently 9th level) in tabletop D&D 5e and without Concentration to limit her, she'd be almost unstoppable. Concentration is a game-balance mechanism. It might not seem to be at lower levels, but when you get more spell casting slots and access to more spells it becomes important.
I do not play PnP but I agree with you.
Concentration is needed for balance.
The problem is that Larian really likes ground effects and AoEs.
Everyone got a grease bottle or acid flask or elemental arrow.
Firebold lets you burn and ray of cold lets you slip.
So its like this:
PnP: Somebody shoots a firebolt at you and misses. You do not take damage and there is no concentration roll.
BG3: Firebolt misses you but the ground is burning. You take damage and you also take damage in the next few rounds.
At the moment concentration makes only sense if the caster can hide behind a wall or all enemies are already engaged by party members.
Prof. Dr. Dr. Mad S. Tist 
