A "user" shouldn't be able to drag an app to the trash to delete it. Because they should be on a limited account, not an administrator account. OS X also keeps settings, in the Library, but they are all small files like com.apple.itunes.plist, etc. Not all bunched in one big mega file thing that can fall apart.
Not that uninstalling on Windows is hard, from the average user's perspective, just deleting the app is more intuitive than a whole process like running an uninstall program.