Because as you think about creating your video (which is almost necessarily many times bigger than any image in file size) you're supposed to think: "what about all the spoken word and textual content of this video file, why can't I access it like any other online text?" in the same way you do today when confronted with large amounts of text in an image, but strangely people don't think that today about video because video accessibility isn't part of the standard web practice (yet).
