Other than their recent history of user-hostile changes (like removing the dislike count), they have two more incentives:
1. If the user cannot see the upload date, they are less likely to realise that the video isn’t recent. This makes it less likely that someone will see a video that they think is “dated”, regardless of reality. YouTube is fighting to overcome younger competitors like TikTok, so it makes sense they would sacrifice some features to claw back the audience that could leave for such a competitor.
2. An upload date is a field that has to be maintained, tested and supported across the entire stack for as long as it’s there. If they think they can save money by dropping support for it (and eventually phasing it out entirely), they are likely to eventually do so.