It seems like work because it's done at work. An aspect of an academic job is that once you've done your 40 hours, you are free to use your employer's facilities, which include physical infrastructure but also library access, and the "branding" of the university to promote yourself. This is what provides that extra utility.

A line cook at McDonald's, if faced with what to do after putting in 40 hours, could at most make more burgers. Many white collar workers are restricted by things like confidentiality and conflict of interest rules.

Many of us get our super utility out of hours 41-60 by having a side business at home.