The excuse is simple: Microsoft is completely disinterested in maintaining two disjoint account management services (complete with all the security, auditing, and attack-surface headaches that entails).
Same reason one's YouTube login is their Google login is their Blogger login.
I held on to my youtube-only YouTube account for a long time, until I realized they'd shadow banned me and my comments to creators weren't getting through, for no reason that I can think of.
Not exactly. G+ was contemporary with / the spearhead for that initiative, but the desire to reify login was something Google was planning to do for awhile; they just used G+ as the focus project around which to pin the initiative. Nobody at Google was really thrilled with having three (four? five?) credential stacks, all with their own bespoke tooling, concept of what an "account" was, security stories, and administrative needs.
(... 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).
And yet it is ridiculous that I have to respond to a 2fa on my phone just to watch youtube on my laptop, every. time.
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.
Same reason one's YouTube login is their Google login is their Blogger login.