Because it's a markup language, and a style definition language respectively, so HTML / CSS lack the high level features and components found in desktop UI frameworks such as Cocoa (standard buttons, lists, file explorers, image grids and such). Building a HTML / CSS UI is done from the ground up with basically nothing more than completely unstyled boxes, in contrast to composing higher level components.

On the web the role of desktop UI frameworks is distributed between third party frameworks such as Bootstrap which is great because those are optional.

I haven't worked with the windows frameworks you mention to compare directly to them.