Because Google steered users this way via their auto-play next video feature. I’ve avoided it like the plague it is so I’m not precisely sure where I saw it, but I’m absolutely sure there was language to the effect of “we pick the next videos automatically based on your likes and dislikes” which translates to “dislike means give me less of this” which means they’ve skewed the usefulness of the metric. It’s no longer clearly indicative of bad content, it’s now indicative of bad content or content users don’t want to see more out automatically.