Due to legal reasons, and to keep advertisers happy, YouTube is forced to display the “Advertisement” mark and a link to the advertisers website. With these, all the required information exists to allow an adblocker to skip any ads embedded in the video stream. No community flagging of ads is required.
If they don’t link to the advertisers page, they’ll lose advertisers, which is the last thing YouTube would do. Legally, a video-embedded “Advertisement” indicator could work, but the link to the advertisers page remains.