When I load a video in "Subtitle Extractor", how come there are no subtitles to extract?

Only soft subtitles can be extracted. Hard subtitles cannot be extracted as they have become a part of the video image.

What's the soft subtitle: Soft (also known as softsubs or closed subtitles) are separate instructions, usually a specially marked up text with time stamps to be displayed during playback. It requires player support and, moreover, there are multiple incompatible (but usually reciprocally convertible) subtitle file formats.
- from Wikipedia.com

Waht's the hard subtitle: A Hard (also know as hardsubs or open subtitles) is a subtitle that is rendered to the video, which has become a part of the video itself and it is impossible to get it off without affecting the video. The good thing about this type of subtitle is that you don't need player support because the subtitle is already part of the video.
- from Wikipedia.com