You can answer this question by asking another: Why is Microsoft so bad at UI?
You have to give Microsoft credit where it's due: it's not just UI they're bad at. Branding, authentication/authorization, gaming, privacy... they're bad in many other ways besides UI.
I've been getting this thing on GitHub lately where none of the CSS renders for about two seconds whenever I load the page. It's not impossible it's a browser issue on my end, but still extremely odd. Whatever they have going on over there is quite a mess. It's a shame, when I used to live in SF, GitHub was THE place to work.
Because Microsoft bought them. And the CEO stepped down. And how the report into Microsoft's AI devision (Copilot).