Keeping your own server(s) up, including spam control, is work that takes time and costs money. Microsoft is probably still cheaper than that,[citation needed] and has more resources to keep everything running than even a medium-sized government. So it is understandable.
Yes. M365 gives "free", secure emails in a scalable, hands-off way. The downside is it practically being a black box, making email issues undebuggable.