Costs are the biggest element in deciding to build tunnels but there are other problems associated with constructing tunnels fo university settings. Back in the seventies, the University of New Brunswick built a huge complex of tunnels between buildings and dorms, but they were eventually closed to the public (https://atlantic.ctvnews.ca/calls-for-underground-tunnels-to-re-open-in-fredericton-1.716917). I had a friend who went there before and after they were closed and she told me that the official reason was an unusual number of cases of rickets (a skeletal disorder that's caused by a lack of vitamin D) since they have long winters and many students never went outside for months at a time.
But she also reported that since many of the tunnels were quite isolated, the real reason was due to the high number of sexual assaults that took place (http://www.uer.ca/forum_showthread.asp?fid=1&threadid=22359). In the Montreal underground, most tunnels have high traffic, from one mall of high rise office to another but in the case of universities, you are literally talking about kilometres of isolated, little used tunnels (https://www.huffingtonpost.ca/entry/tunnels-skywalks-campus_n_6547918?ri18n=true).
In either case, it's always good to get a bit of fresh air, even if just to cross the street.