The Past Tense Is the Punchline
"You are fake" is an accusation. "You were bought" is a receipt.
The tense does the work: "you are fake" leaves room for debate - the person can argue they are not. "You were bought" is a completed transaction. The loyalty is not up for negotiation; it was purchased, and the seller is not in the room anymore. There is nothing to argue with, only to point at.
Why it crossed the AI space: the meme started with the assistant who is nicer than they should be to a prompt - "you were bought to be pleasant." But the phrase is not about AI. It is about any loyalty that is performed rather than held. The AI version is just the cleanest one, because the buyer is visible.
The format: the line is short enough to be a comment, a sticker, a reply. That is why it went viral - it is a two-word accusation that fits in any thread. The share is the pointing, and the pointing is the joke.