Because Figma does not give a plugin the video itself. For a video that is already on the canvas, the plugin API exposes only a fingerprint, not the file. So Video Exportify asks you for the file once more, from your disk, even when the same file is sitting in the frame. The layer still decides how the clip is cropped and placed.
The quick ways to attach
Press Upload video on the layer, drop a file onto it, or drop several files onto the list: they land by name, then by order. A clip you have already uploaded can be reused from the menu next to the button.
Try it on your own frame
Free to try, no account. Or watch the plugin work in the browser first.