It's been a while (~10y) since I administered windows servers, but I recall they were much harder to automate w.r.t. maintenance. So Microsoft created M365 to fill that niche and have organizations offload operational cost to them.

With a linux fleet can run via SSH/ansible and you can patch thousands of machines programmatically in minutes and control the deploy at a fine level. With Microsoft/Windows it often involves/d logging in to the one AD server where everything runs, back it up via 3rd party software, apply the patches and hope it doesn't break, and if it does, call MS support, because rolling back is really your last resort (I feel it's become better since 10y ago, but I don't have hands-on experience anymore).

Our company runs on-prem Exchange since 2019, and... well... it's a PITA to work with (Legacy Outlook/New Outlook), we downgraded from M365 and it was really a step down in terms of features and reliability. OTOH I guess it's keeping us afloat today when M365 is having a really bad day.