Running your own local AuthN/AuthZ is more than just 'install it on a box in the closet'. I don't blame anyone for letting one of the giants do this on their behalf -- they have the expertise, though I agree I wouldn't touch Okta.
This isn't email.
Yes, you need someone to wear the IAM admin hat. But once you get it configured and running it requires 0.1 FTE or less (likely identical to whatever your Okta admin would be). Not worth 6+ figures a year and exposure to Okta breach risk.
Yes, creating a SAML integration is easy, but that's only one piece of the puzzle.