Because, well, once upon a time there was Exchange Server and MAPI (the "Messaging API"), and that actually worked pretty well, and pretty much everyone started using it (or one of its EWS/ActiveSync offshoots), but then the Internet happened and ruined everything, but we got MAPI-over-HTTPS eventually, which by that time had grown so many warts that nobody at Microsoft really understood it anymore, so that got deprecated, even though, well, everyone and everything still uses it.

The new hotness was Azure-slash-Office-365, and that's where the "new Outlook" web interface lived and (sort-of) continues to live, and the on-prem customers got stuck with the "Exchange Subscription Edition", which was basically Exchange 2019 plus security hotfixes at extortionate rates, but with none of the new functionality.

And, since MAPI continued to be really, really hard even in the age of Copilot, access from the new hotness to the deprecated on-prem servers was implemented via IMAP (LOL), but force-proxied via the outlook.com cloud/clownshow.

Which is why, right now, many Microsoft customers that thought they were self-hosting their email are entirely down. Lessons might be learned here, but I sort-of doubt it...

None of the admin ran shops are down 1 day per year.