The problem is, 99% of the time the person getting rid of the application just clicks the "Uninstall" routine, and the developer who wrote the application did a piss-poor job of writing the uninstaller, so it leaves a bunch of garbage behind. Or the application is from a dubious source (many "free" apps you get online) that doesn't want to be uninstalled.
It's easy to completely get rid of an application on Windows, so long as you know what you're doing. Just like it's easy to root and a phone and uninstall all of the bloatware, so long as you know what you're doing. Notice the trend?