I agree with the other's regarding the dynamic nature of the content. In addition, the widgets themselves haven't changed all that much for desktop GUI's. WxWidgets, Win32, Gtk, and Qt have mostly the same basic set of controls today that they had 15 years ago. Just imagine the extra R&D churn it would consume if you also had to update your GUI editor every time you changed a web UI control!
But you can create a custom control (not as nicely as Delphi has that covered) which can be packaged and documented and then reused. With web, because of CSS this is a problem; shove your custom html control in another page and it suddenly looks bad, not because of a few colors and a bit of alignment (like it would be on the desktop), but because of different framework use, different layouts, clashing css etc. It is a pain compared but he, it is what it is for now.
