When the number of students doesn't match up with the number of placements available, some universities encourage their students to accept unpaid co-op placements. When I was dean of the engineering faculty at Sherbrooke university, I would not allow my students to do unpaid placements. I would tell industry representatives that they couldn't have my students' services if they didn't want to pay them.
One year, we had a real discrepancy between the number of placements and the number of students. As dean, I did something highly unusual. I had to invest money in order to create jobs within the faculty for those students who had not been placed. Situations like that happen. It goes in cycles, so there are gaps when it can happen. It all depends on the year.