Because on Reddit (and here on Lemmy) people use up and downvotes exclusively to rate the quality of a post, not as a tool to show disapproval without having to be able to actually articulate why. I like when social interactions go exactly as intended - like in this case - and don't devolve into two people arguing with silent mobs behind them.
/s for everyone who's as blind to sarcasm as this shitty AI from Google.