UIKit after four iterations (i.e., iOS 6 in 2012) was far from its "very early days": it was a well-designed, production-ready, powerful UI framework that was way closer to AppKit than SwiftUI (in its fourth iteration) is to UIKit now.
UIKit started out with more polish because they took the AppKit they had iterated on for so many years and slimmed it down for extremely resource constrained devices.
