It may have a lot to do with the fact that websites tend to be more verbose in their instructions, so that traditionally website have had their documentation in place.
For example a desktop app may ask for 'user_id:' where as the web app would ask for 'Your email address:'.
There is alot more space on a web browser's screen for this sort of thing than in an apps dialog box. This does not excuse the mindset but does partly explain it.
This does nothing for the underlying complexity though, if the user does not know what the app is attempting to do then they will probably not be able to use, regardless of the UI.
--tidiness is the memory loss of environmental mnemonics