In many places, post-secondary state funding has been cut (and public universities thus rely much more on out of state students who pay more.)
At state schools this explains the vast majority of the increase in tuition. An estimate from 2015 pegged it at 79% of tuition increases just going to offset decreases in state support for the universities.*
At both private and public schools there has also been a lot of competition for students based on lifestyle factors. This means a lot more spending on fancier dorms/student fitness centers/etc. This doesn't show up in increased tuition, but increased mandatory student activity fees, increased meal plan costs, and increased dorm costs.
Education also suffers from "
Baumol's cost disease." This is a pattern where when per worker productivity goes up a lot in some parts of the economy, the parts of the economy where per worker productivity cannot increase get more and more expensive. We can make an factory worker much more productive by investing in more advanced automation (and have been steadily doing so for decades), but it is not clear how to increase productivity per professor through capital investments.
*https://www.insidehighered.com/news/2015/05/05/report-says-administrative-bloat-construction-booms-not-largely-responsible-tuition