The entire basis of spellcasters is predicated on certain assumptions about concentration. It assumes spellcasters use only one concentration spell, and then non-concentration spells being used while the concentration spell is up. It assumes spellcasters won't risk melee range, because they risk losing concentration if it. It assumes spellcasters take feats and pay attention to Constitution to try and keep their concentration up if hit. It assumes concentration spells will be exceptionally more powerful relative to non-concentration spells because of these elements.