There's usually both a time and severity limit to what magic can heal. It works differently depending on the system, but generally the longer it's been since the injury or the worse the injury was, the more advanced magic required to fix it. You can't just dump more magic on it either, it's gonna take more talented spellcasters with specific skills, e.g. the difference between someone with first aid knowledge and a trained neurosurgeon. Bad enough and you're getting into "there is literally one person in the entire world who can do this and they're busy" territory.

That's assuming it's a simple injury and not a curse or the like. That's also assuming it's not a disability from birth; regeneration isn't going to do a damn thing if the body's natural state is lacking a sense or an appendage.