The group of students I came with was pretty much ready to go with 5 minutes left in the 3rd, but one girl really wanted to stay for "Do the Gopher" at the end of quarter (of all the reasons to not want to leave the game), so she got to do the Gopher then my entire group except for me left before the start of the 4th quarter. I had a bit of a desire to leave since we seemed to have the game well in hand halfway through the 3rd quarter and the game felt like it was taking a long time. Being a Gopher fan, however, I knew that there was at least some chance of a Purdue comeback, and so I stayed until the end and sang the Minnesota Rouser and Hail Minnesota.
As some other posters mentioned above, a lot of students, and perhaps others, aren't there because they just love the Gophers and are thrilled to be able to cheer them on in person, they go because it's something fun to do with friends, and because they "knew" who was going to win about halfway through the third, they left because they didn't think there was any reason to stay. Plus it was getting cold, my feet were pretty numb by the time I started walking back to my dorm.