The SSO "market" is only a market if the SSO providers are monetizing your data at the expense of your privacy. Private authentication is not a "market" almost any of us needs or wants, but rather a right that we deserve.
Not sure why you've put "market" in quotes there. Okta is literally a SSOaaS that has a $14 billion market cap.
Does anyone use Okta outside of the Enterprise? No enterprise focused app is going to allow any random third party credentials. Okta is also not by any definition a “social network”.
It's also a "market" if you happen to run one of the largest payment networks in the world (Apple Pay) and if having users signed in with your service gives you the ability to let them easily provide payment information through you to the exclusion of your competitors (Amazon Pay, Google Pay, etc).
Apple Pay is a feature of the iOS SDK and Safari browser, not a feature of the login system. Which method you used to login doesn't change the friction of paying with it.