The issue with the “bring your own widgets” approach in my experience is that it’s unusual for third party widget packages to have anywhere near the sort of depth and breadth of capabilities compared to the same widgets that come OOTB with native UI toolkits. This means “shopping” for the widget with the best tradeoffs for one’s particular use case and making up for whatever inadequacies it comes with yourself (or just living with the inadequacies, resulting in a crappier app), which is annoying at best.

It’s so much nicer to just be able to trust that an extensive library of stock widgets will be able to do whatever it is that you need, as tends to be the case with AppKit/UIKit.