They’re copying iOS behaviour

Agreed, but it should change. On iOS the OS can kill the browser for you so you may still want that page you didn't kill.

On the OS quitting the browser should reset it back to a normal state that doesn't include the private tabs you had.

I don't really see why a distinction would be made there. iOS's and macOS's process models are being made more similar as time goes on. And as a heavy user of both platforms I'd find it surprising if it didn't behave this way. An option to change it would be welcome, though.