I can tell you that despite what you might figure, buildings really do get used. I went to a massive university with 50k people, and a campus so big it took a half hour to traverse yet was packed with massive buildings the whole way. For the most part the buildings would be booked up during the regular term, but during exams, then they were absolutely maxed. All the study spaces in the 13 story main library, 6 story engineer library, and all the other scattered libraries would be full. There would be no free chairs at all. People would resort to studying in empty classrooms during finals, which usually worked better since you had a closed off room to meet with your group, space, AV hookup, whiteboards, etc.
If there wasn’t a need for more and more rooms in a college, they wouldn’t get built. It’s not some conspiracy.