Because it is a project file, it does not contain raw video frames but rather to render a final video.
If you want to edit the video or upload it to YouTube, you don't want the .mpl file. You want the source video. .mpl video file
If you have a high-definition camcorder (Sony, Panasonic, Canon) or have ripped a Blu-ray disc, you will see .mpl files. Because it is a project file, it does