That's because that's how dogs communicate naturally. They don't sit around in the pack barking at each other; it's ALL body language. Most training programs/regimens (obedience, agility, shutzhund, sporting.... big list on:

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rely heavily on hand signals & body language. Dogs spend much of their conscious time studying/watching us to pick up on our cues, which can make it seem like they're psychic/telepathic. They're also very ritual oriented, which has a lot of connotations....

Doc