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 didn’t label their videos as paid promotion adequately. On the bright side, it’s 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, it’s 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 that’s in a way that’s hard to miss, the “includes paid promotion” seems more like a tool for those who DON’T 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 don’t disclose at all, that’s up to the FTC. And the only real way they’ll know if someone reports a video to the FTC directly as a complaint On their website. It’s up to the creator to do what’s right in terms of transparency and disclosure.
The ONLY thing that will completely satisfy the FTC requirements on this is a clearly worded vocal statement in the video itself about the nature of the relationship in regards to the product. For example. If you got the product for free, you would say, "I got the product for free". If the company is sponsoring your channel, you would say, " This (Name of Company) is sponsoring this video" or (is a channel sponsor".
The issue is that there were tens of millions already on YouTube long before the FTC invented this rule. So, youtube provided the ability for the creator to turn on the Paid Sponsorship, notice for those old videos in which the creator did not make an vocal statement about the nature of their relationship to the product manufacturer or seller. However, turning this on, is not to be considered as "Fully Complying" with the FTC rule. It is not. It is kind of a "HALF MEASURE" so to speak, to show that you tried. The only real way to fully comply with be for youtube to allow the creator to edit old videos where they could break in and make a vocal statement, sort of like and AD Break. Sort of like, "Hey, just wanted to let you know, I got the product in this video for free". Or they could allow the creator to REPLACE old videos with an updated version of the same video, without losing that video's position and ranking.