Windows can unzip files from inside a dialog, which is convenient if files are zipped and you want to unzip.
Can Ubuntu do this? If so, how? If not, why not?
No. Most of the dialogues are limited to just changing the sorts of things they can show although I should point out that some dialogues work in different ways.
Ubuntu uses GTK for most file dialogues so no, you don't get full context menus.
As for "why not" I can only describe how it's built. The dialogue is just a cut-down list of the files. It's not designed to be a full-on interface for files. In Windows each file dialogue is an embedded instance of Explorer.
Ok it is maybe not exactly the same than on windows but if you start nautilus (the file manager), browse to the location of the archive and then right click on the archive name then open with the archive manager, then you have access to more or less the same functionality. The good news is that the archive manager can open most of the archive types (can you open a rar file into winzip ?). In others words you can do more or less the same using the mouse.