Some customers require that your employees have to comply with their password requirements. Using an IdP means there is just one password to worry about, but you still might have to periodically change your password because some big customer has it in their contract.

And then there are customers who have requirements about passwords, even though they are only going to be using an IDP anyway. It is probably because they have the same standard list of requirements that they give to everyone, and probably was copied from somewhere else, and you can usually contest it. But it is still a waste of time for everyone.