In Sanderson's books, having magic powers generally indicates or causes a high place in society. You have the Elantrians ruling Arelon, the nobles in Mistborn are supposed to be the only ones with powers, the Returned in Hallendren are treated as gods, etc. We know from Kaladin that, progressed far enough, light eyes appear. It is not unreasonable to assume that at some point this becomes permanent. Since the Knights Radiant were superheroes in every sense of the word I can think of (Reckoners doesn't count) it makes sense that they would naturally get power.
If nothing else, those with Shardblades usually beat those that don't.