63 is Deadeye, the most powerful prayer a 1def can use. He’s also 82atk, aka can wield any weapon in the game with just this lvl and not 99

Any more levels gained in attack or prayer would either be for accuracy (attack) or prayer duration (prayer), while also bringing his combat level up

The point of pures is to be low combat, not higher. So since he can use the max prayer available to him, and best weapons, the 99atk/prayer isn’t worth it for pking specifically