The "Microsoft Tax" is often cheaper than the "Linux Engineering Salary." While Linux alternatives exist, they require "assembly"—integrating LDAP, Kerberos, DNS, and config management (Ansible/Salt) to do what AD does out of the box.
Most businesses don't want to be in the business of maintaining their own identity infrastructure. They want a utility. Between Group Policy’s granular control over the endpoint and the tight integration with Exchange/M365, Microsoft has created a "sticky" ecosystem. I've tried the "DIY" route with Linux mail servers, and the friction of maintaining deliverability and security patches manually is a nightmare compared to the "it just works" nature of the Microsoft ecosystem.
I am not a system admin, so maybe this is a crappy take.