Because some seem to have not paid much attention to what was going on somewhere around 2017/2018 when the FTC was threatening to crack down on creators who didnΓÇÖt label their videos as paid promotion adequately. On the bright side, itΓÇÖs a lot easy to fix then the alternative which is to have a burned in text. FTC made another big deal about it a year or so again when they not only focused on youtubers, but also Instagram and Twitter influencers.
At the end of the day YouTube has provided the tools, itΓÇÖs up to the creators to know the relevant laws around paid promotion.
apparently mentioning the sponsor at the start is enough to satisfy the FTC. The ftc want it known from the very beginning of the video thatΓÇÖs in a way thatΓÇÖs hard to miss, the ΓÇ£includes paid promotionΓÇ¥ seems more like a tool for those who DONΓÇÖT have their disclaimer in their videos at the start. That way you can easily add the sponsorship disclaimer at the start of videos before the FTC requirements were a thing without needing to take down or edit the video.
As for videos that donΓÇÖt disclose at all, thatΓÇÖs up to the FTC. And the only real way theyΓÇÖll know if someone reports a video to the FTC directly as a complaint On their website. ItΓÇÖs up to the creator to do whatΓÇÖs right in terms of transparency and disclosure.