A lecture can potentially contain three things, all presented by the lecturer: images, video and audio.
Did the lecturer use any copy written material in a lecture?
Does the lecturer have releases or licenses for each of these items in their lecture?
Who makes sure the lecturer did not 'cut corners' in creating their lecture, by using content that was not theirs to use, legally, without proper licensing or attributions?
What happens when the lecturer offloads the creation of lecture material to a TA or someone else, and they use content that they are not legally allowed to use for whatever reason?
Should the lecturer be held accountable?
The TA/whoever created the lecture content?
The school the lecturer works for?
Just like why there are a lot of great old movies that cannot be found on VHS/DVD/streaming: At the time, the proper licensing was not obtained, or even thought of yet(take streaming for example)! And going back to secure those licenses costs money, sometimes a lot of money.
Yes, it all boils down to money. Doesn't almost everything? :-/
The most likely lectures that could be released, IMHO, are chalk-talks(all content is written on a blackboard), as long as the lecturer does not want to be additionally compensated for the replaying of the lecture by others.
You would think that if a school pays a lecturers salary to teach a course, that would be the compensation, but in a lot of cases, the lecture the school paid the lecturer to create is actually owned by the lecturer (due to an agreement with the school) so the lecturer has final say over what is allowed to be made publicly available.
And really, who would pass up a chance at even more money? Not most lecturers! But in some of those cases, it is just not worth it to pay the lecturer for public display of the lecture, so the public misses out, and the lecturer ultimately gets nothing additional for their greed.