I think that’s countered a bit by less of a need for custom widgets. In fact I’d argue that one of the big points of appeal for native Mac apps is that they eschew tons of custom widgets in favor of the standard system kit whenever possible.

There’s also a good deal of power that you gain when writing custom Cocoa widgets; drawing is usually done with Core Graphics, which is a blazing fast, extremely capable C API. There’s a lot you can do in custom Cocoa widgets that would be impractical or dog slow with HTML+CSS+JS.

EDIT: Also, one big, big difference between WebView apps and AppKit apps is that while WebView apps might get their appearance reasonably close to native, they almost always miss the mark entirely when it comes to widget behavior.