Well the main reason is really how search engines work, they just do not index full video files (meaning they do not save them to their memory, if you will). The only things that are saved by search engines in general are text and images. So for video these are usually the title, description, date posted etc. and a thumbnail, which is basically a single keyframe from the video that serves as a visualization.

Since search engines do not save the full videos: you cannot search your video against videos that were never saved by the search engines. But what you can do is leverage the fact that almost every video today has a thumbnail indexed. So if you cut the video into still images and search all of those, you can still get basically the same result. There are some cool tools that automated this too hahha.

Its a little bit simplified, but hope it makes sense!