One of the worst things I've seen at an employer was more than one SSO. That's when you know the people paying your wages are truly dysfunctional. Because of course Fiefdom A doesn't want Fiefdom B's SSO to be used by their systems, that gives the person ruling Fiefdom B power - so instead get a separate SSO, what do you mean that's missing the point?
If I worked for an SSO provider I think I'd be tempted to expontentially scale corporate pricing. Want just one SSO system for (say) 10 000 employees? That'll be $5000 pa. Oh you want a second one at the same company? $20k pa please. Third one? $100k.
My reasoning would be: An outfit which buys more than one is completely irrational and so they are my best target to gouge as heavily as possible because they'll be least able to do anything about it.
I’ve mainly seen that situation in post M&A environments, and while fiefdoms are often a part of the problem, untangling auth across multiple enterprise systems is really hard.
The fiefdoms I'm thinking of were created naturally inside the organisation, not acquired. Somebody's power grows, a department with ten people becomes a business unit with a thousand people due to its success, other managers resent that, and senior management mistakes this as an opportunity for "internal competition" rather than a dysfunction which needs to be crushed, if necessary by terminating those who promote it.
Lots of things are hard about M&A but it's made easier if people remember from the outset that their business exists in the world. You don't have and can't build a business which functions independently from the world, stop trying. On the technology side that means things like using FQDNs in URLs (https://webinar.mycorp.example/2020-New-Products not https://webinar/2020-New-Products)
I think that's telling