Why wouldn't 1Password implement their own SSO rather than use Okta?
1Password might select a different vendor or self-host an open source or commercial SSO system. Still, there are no perfect answers, and each choice has tradeoffs. Even worse, the new system is not guaranteed to be any better than the old system.
One thing which frustrates me when reading a lot of these posts is a lot of people assume that security is easy, organizations should be perfect, and breaches only occur if an organization or service is terrible. None of these are true.
It is one thing to outsource a monitoring solution. But outsourcing your SSO to a third party? They can only guarantee how well 1Password works, they cannot guarantee how well Okta works.
I think the problem is trust. So the question is, why would 1Password trust Okta?
More comprehensive "don't share sensitive information with other people, even if they ask nicely!" training would be a lot cheaper.