I don't really know why. But if I turn off ad blocking, YouTube's own ads are way more obnoxious and intrusive. Maybe it's because every advertiser has figured out they need to cram as much shit into a 5-second segment? Or maybe it is because seeing an ad at the very beginning is much more psychologically disruptive (or if the YT ad is in the middle, it is usually inserted without any regard to the video, even breaking up sentences).