re: I don’t get why it is more costly for colleges to educate students now vs the 70s/80s

Expectations for tenure have definitely changed since the 70s and 80s. Back then a faculty member could be achieve tenure without a terminal degree, had more lenient publishing expectations, and could get by with lower teaching expectations and assessment. It was also not uncommon for faculty members in 70s and 80s to be offered tenure based on how they fit into the culture of the department rather than the merits of their overall work. If someone was a "good guy" who was well liked by their peers but a horrible teacher and showed no improvement over 5+ years then they had a decently good chance of being offered tenure as long as they could pass the vote and wasn't unknown to the Provost.

I won't argue that everything is fixed now at academic institutions but to claim that schools give out tenure to faculty like it's candy ignores the just as flawed process that existed 40+ years ago. And issues with tenure go both ways; some professors create openly hostile workplaces, especially with junior faculty, because they know they can get away with it. We just don't hear about about that often because people outside academia aren't interested enough to care.