You are looking from the perspective of a user of the software - sure, these have enough feature parity to "compete".
But that's the butt end of the equation. The real issue is enterprise administration. A user never thinks about this problem, because they do not ever encounter it as a problem in their private lives.
How does permission work? How does a new hire get an account? How does account/permission revoking work? How does audit work? And that's just the surface.
Needs for large enterprises, where you cannot just have John from HR make a new account for the new hire, are often not met by the opensource world.
And decided that it was cheaper and easier to just outsource it to Microsoft. Because doing it in today's environment - different work computers, backend servers, mobile devices, etc - is much more complicated than just managing permissions on a mainframe.
Distributed databases are a solved problem (besides maybe performance). Offloading account management to arbitrary databases too. Why everyone is using Microsoft is, because then they have someone to blame, instead of needing to point at themselves.
And setting up things like rsync to replace dropbox is also "fairly quick"!
The point isn't that but the fact that like a normal user, a normal business don't want to have to tinker with low level components to get the functionality they want. They desire to pay and get a working piece of infrastructure with low hassle (tho i get saying active directory being low hassle is weird).
But a normal user isn't going to setup AD either. This will be done by sysadmins anyway, so stuff like being able to put the configuration into version control is actually useful for them. The "normal business" has lots of employee databases anyways and integration is actually a feature instead of needing to sync it with bespoke Microsoft internals.
So you can hook up all those internal employee databases to your new created libpam-mysql and hook it up all to slack or just use what Microsoft sells you.
I do not need to create it, it already exists. Yes, you can write your own pam module, but in general you do not need to.
> just use what Microsoft sells you.
Which means now your employees need to manually sync the MS and your internal databases. Depends on how much your employees time is worth for you. I mean a lot of companies do exactly that, but it is certainly not the cheaper option.
Also using what MS sells is also illegal. Not that anyone cares, as whole Europe ignores that, but when you meet a civil servant on the wrong foot, your company is toast.
But that's the butt end of the equation. The real issue is enterprise administration. A user never thinks about this problem, because they do not ever encounter it as a problem in their private lives.
How does permission work? How does a new hire get an account? How does account/permission revoking work? How does audit work? And that's just the surface.
Needs for large enterprises, where you cannot just have John from HR make a new account for the new hire, are often not met by the opensource world.