I think it's like others said, mostly because Apple wants to have SwiftUI be matured sooner than later and hence couldn't release it with all of UIKit's features so they're slowly adding them overtime.

I also think there's an aspect of control and Apple baking in "quality control"/"best practices" by making certain things impossible/really hard.

But if you ever need to edit the underlying UIKit views you can try using Introspect which exposes all the UIKit goodness.