Yeah, but those are cheating, and it's not particularly easy to cheat in that specific way. In a high level match, whenever you shuffle your deck, your opponent can (and in fact is required to) also shuffle it, and they're the last to touch it.

Cheaters, like in any game, tend to look for methods that maximize the combination of impact and plausible deniability. Marked sleeves, sleight of hand (pun very much intended) etc.