I'll give you the real reason...
The rules of chess have evolved over time. In the past, different regions even played by different rules. What causes rules to change, and what causes people to agree on rules, is when people realize the game would be better a different way... but what does "better" mean?
Without writing an essay, essentially it means a game that's rich in both strategy and tactics. The rules are the way they are for that reason. Chess isn't trying to simulate a real king, or a real battle, or anything like that.
If the king could capture its own pieces, I think this would reduce the tactical depth of the game (many sacrificial king hunts would be impossible) while not adding anything to the strategic depth.
Why can't the king take a piece if surrounded by guards?