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).
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.