We have law that says "don't advertise to children", and that's quite tricky so if a beer company says "we only placed ads in programmes that aired after 9pm which had adult themes" they're in the clear. If they place ads in shows at 7pm that are aimed at families they're in trouble.
The YouTube version is the channel saying "I have no idea who watches my channel because YouTube won't and can't tell me", advertisers saying "We advertise with YouTube, we expect them to place the ads responsibly, and we can't rely on YT analytics because COPPA means many children watch while logged into a parents account" and YouTube saying "we don't care, it's entirely the responsibility of the channels and advertisers".
It's this last "we don't care" bit that is causing problems. As the watershed example shows you don't have to have a bullet proof solution. You just need to make some good faith attempt to create conventions around ad placement that reduce the numbers of alcohol ads being served to children.