Pike probably has a form of aphasia, lacking the ability to effectively communicate his thoughts and wants beyond simple affirmations (or lack thereof), in addition to advanced quadriplegia and apparent facial paralysis. I also presume, perhaps related to his spiritual upbringing, that he didn't want cybernetic or experimental treatments to fix his motor problems, and it could be that he was little more than a shell below the brainstem, the chair doing all the work in maintaining his heart and lungs (if he even has his original anymore).
Since we see a Pike Chair well into the 24th century now (on Lower Decks), perhaps this is a sign of the regular treatment given to what would otherwise be vegetative states. The brain is restored as best can be, and the rest of the stuff controlled via the chair, and they are either taught (or in Pike's case, never lose the ability) to live on their own, albeit in this contraption.
Since we see a Pike Chair well into the 24th century now (on Lower Decks), perhaps this is a sign of the regular treatment given to what would otherwise be vegetative states. The brain is restored as best can be, and the rest of the stuff controlled via the chair, and they are either taught (or in Pike's case, never lose the ability) to live on their own, albeit in this contraption.