[asr]

My (uninformed and possibly wrong) guess is that YouTube wanted to maintain the fiction that, to the best of their knowledge, everyone using the service was the person who signed up, so that it could argue it was not intentionally showing videos to children. When YouTube knows kids are using the app, it's supposed to be doing lots of things to protect their privacy (which it has not been doing).[1]

Building an algorithm to guess when children were likely using YouTube on their parents' account, in order to show age-appropriate ads, would show that YouTube does know that kids are using the app. So my (again, uninformed and quite possibly wrong) guess is that YouTube did not try and fail at creating such an algorithm; they deliberately decided not to try.

[1] https://www.washingtonpost.com/technology/2019/07/19/ftc-app...