Respectively, I disagree with your argument.
How do you handle storyboard previews when doing UI in code? It’s one of the strong feature of interface builder. Let’s say if I want to preview iPad Pro in full screen, split views, or viewing in smaller screen device like iPhone SE, I can preview it immediately using interface builder that makes our development time really fast.
Secondly, UI in code leads to massive view controllers. View Controllers in UIKit applications are already huge.
Also, interface builder provides warning for conflicting autolayout constraints & suggest fixes. Whereas UI in code warning of autolayout constraints is thrown is console which could be unintentionally missed with other chucks of logs.
I do agree, Apple can do a better job in create separate storyboard for each view controller instead of a giant storyboard XML that can lead to conflicts, but individual .xib can be still an option.