This is obviously not the reason why. There is no reason for universities to send out rejections until they are comfortable about their odds of ensuring n number of (the best possible) matriculations. In particular, universities currently have noisy information about the expected outcome of any applicant because competing offers have not been made or communicated yet. To answer the forthcoming question: "why not simply waitlist applicants?" Waitlisting is still difficult because waitlisting only makes sense when universities have a reasonably precise understanding of their expected cohort quality, i.e., how do you know who is "almost good enough" if it is not yet known who is "good enough." As for the final question: "why not reject the clearly inadequate applicants?" Universities already do this, but some don't. The ones that don't correctly understand that it is still a very small risk. There is also some level of effort that needs to go into rejecting individual applicants (set up the portal stuff and send out emails, etc.). In sum, wait for MIT, Harvard, and Stanford to finish eating.
