SSO is important for enterprise.. but github presents a challenge where devs usually have pre-existing personal accounts which are added to company github accounts, rather than the company account being used.
I've also been the admin on those Business accounts. It's easy.
It makes sense to separate access ("this person has access to Example Corp's repos only as long as they work for Example Corp") from identity ("this person owns this account").
Introducing single-sign-on as one way to simplify login, and potentially as a second-factor for gaining access to repositories run by the business, makes sense. Making developers create entirely separate accounts doesn't.
There are perfectly valid reasons for segregating accounts so that there is complete separation between them.