I don't get it. Don't they already have the signal available to them of "did the user watch all/most of this video, or did they abandon it early" ? That seems l

I don't get it. Don't they already have the signal available to them of "did the user watch all/most of this video, or did they abandon it early" ? That seems like it does most of what they want, and thumbs up/down will add only very little information on top of that?