Well, lots of reasons. For starters, you have to consider the privacy of every person who appears on camera, including students and guest presenters. But let's assume a blanket waiver can be put in place (ha, good luck). Then you have to consider the chilling effects: will people want to have open discussions in lectures (for instance, to challenge a professor) if such questions (which may be read as dissent, or picked up on by outsiders to smear the speaker) are de facto recorded?

Then there's copyright, as university lectures may contain copyrighted video or audio that the university has licensed for educational purposes but not retransmission.

Let's also consider intellectual property, because guest speakers (who may have been paid for their contributions) may not be able to get speaking gigs as easily if a university has put their lecture online.

From a technical perspective, you also need capture and storage equipment. That's not just cameras and hard drives. You need someone to operate the equipment (e.g. framing shots, checking audio levels) because a catch-all wide shot of a lecture theatre is not very compelling or engaging. And all material has to be catalogued so it appears in the right place.

Many universities ARE taking the opportunity to provide lecture recordings because it burnishes their brand and provides a public service. But let's not pretend it's as simple as hitting record.