I think I'm the most ridiculous, but for the same career reasons as the rest:
Active directory, yes, plus: Azure cloud sync with entra active directory Hybrid exchange on prem and office/exchange online.
For better or worse, large enterprise isn't going away from M$.
Also, I have transparent proxy sophos IPS, security Onion IDS, Trellix ePO, and other security products all being integrated for info security testing.
Not suggesting this is normal, just my test/dev playground I don't have to worry about breaking.
Sometimes it's for career progression or familiarity.
Just for SSO, might be easier ways, sure.