You have a good point, but I would point out that deaf people watching a silent un-captioned video is more access to knowledge than watching no video at all.
Watch https://www.youtube.com/watch?v=Ei8fyGwsy1o without looking at the captions and see how much you get from it. You're not going to get much if anything from the video. Now, add a professor who will be turning to write on a blackboard, doesn't move their lips enough for you to read them, etc, and you can start to see how this is a real problem.