Same reason one's YouTube login is their Google login is their Blogger login.
(... the one thing that came out of G+ that has stuck around is infrastructure improvements. A lot of the stuff the G+ team wrote to support Google+ behind-the-scenes is still used at Google for other projects now).
I have a paid yt account fine, but watching trivial videos is not in the same class with accessing my gmail, or managing my gmail, or even managing my own videos if I were a youtuber. I'm fine with being one identity (well no I'm not but pretend I am) but those are not all the same activity or class of activity.
I mean, they did get my money so I guess why should they care, but I have no idea how people without an actual computer science degree and mumble years of experience un-fucking broken computer horse-shit manage to get through the process.
[EDIT] Oh, advice to any parents considering embarking on this: just lie about your kid's age. Don't try to use their child accounts. It makes the whole thing a much bigger pain in the ass.
And while I'm at it, may as well ask: anyone got a good list of allowlist addresses for Minecraft? I lock down my kids' laptops (you know, like a responsible parent is supposed to) but modern Minecraft is so full of friggin' spyware that it can't even launch without connecting to a bunch of never-seen-before IP addresses every time it starts, which means I have to go put in a pin multiple times to allow those new addresses every single time.
At this point it's just really a pain to make a server and have some of my sons friends join it. The parents need an MS account, then need to associate with the kid, they need to approve things that seems like you want to show their kids a gory horror film. It's like MS is the parent, or MS thinks I'm a bad parent. Come on, I'm the parent, I'm perfectly capable of guiding my kid online.
I don't do it anymore.
I've always assumed this is just a dark pattern. There's no excuse.