I don’t think YouTube normalizes the audio on their videos. I have no idea why, but that could easily become a quiet video that leads into a -16 LUFS ad that blows your ears out.
Supposedly they'll turn it down if it's mastered louder than -14 LUFS. They won't turn it up if it's quieter.
I couldn't find a Youtube source for this but it's mentioned extensively online: https://audio.rswaver.com/blog/youtube-loudness-standards
They have a "stable volume" toggle, actually. I don't see ads, so I don't know whether it works for those.
Because content creators have a right to produce audio how they want.
If it’s a video of someone speaking in a quiet voice it shouldn’t have to have an average volume level of a new Metallica video. Forcing people to use heavy dynamic range compression to get it there would not be good.