Why do most websites have a custom skin where as most desktop applications don't?

You rarely ever see websites with a UI that has the same skin as the operating system. But, desktop application rarely ever change skins. Why do custom-skins work so well for webpages but not for desktop applications?

Answer

I think that is an historic thing, newer desktop apps are actually very likely to have a custom skin.

I think the reasons are:

1. 
   

   Web browser's default style is useless (it's ugly and not very usable) and differs between browsers so you can't actually use it.

   
   


2. 
   

   Web sites (and as an extension web apps) are much more likely to have an graphic designer involved in the development process than desktop apps (this is probably no longer true).

   
   



3. 
   

   Desktop app's controls are extremely hard to re-skin (with newer platforms this is definitely no longer true)

   
   

I personally develop and sell a desktop app with a custom UI.