Because students don't know how much a company wants them, and so cannot set their ranking with that knowledge in mind. For example, Apple could rank a student a 9 and Microsoft a 2, but to the student it looks like their level of interest is the same. The student could personally prefer Microsoft, but have a low chance of getting matched, whereas if they knew Apple was closer to a sure-shot of having a job, they might be more risk-averse and rank Apple as their most desired over Microsoft.