They're not hiding anything. Obviously George Washington used a time maching to play Zelda games. He liked them so much that he put a triforce in every E on American bills. The government's not keeping this a secret but for some reason people are still wondering why.
The fact is, the symbol for three triangles was used long before it represent the triforce. Generally in entertainment like movies, games, books, etc... when you bring up actual things from history, you get more props as a creator. Thus the three triangles was most likely placed in Zelda because of its continual appearance throughout history. I believe the first appearance has to do with something spiritual, but I'm not entirely sure. The triforce is not the only symbol. Zelda has incorporated many symbols from history and I'm pretty sure it isn't going to stop.