OpenBSD does not need GSoC to attract contributors. The project gets a good amount of new contributors on a regular basis, and they get onboarded quickly without causing much distraction, if any.
The mentor/student relationship is atypical for open source projects which are used to operating as a community of equal peers. Mentoring students who expect to be mentored takes a lot of time, and the vast majority of them don't come back. In my experience money is a key incentive for students in GSoC and that makes it hard to keep them as volunteers. Unless you are very lucky as a mentor and pick a student who turns out to be an open source enthusiast, they won't actually care about your project in the long term. And there is no way of knowing that during the application process. Unless in special cases where you already know the student, as I did in one instance, but that's an exception.
(Speaking as an OpenBSD dev, and as a former mentor of several GSoC students, over several years, at the Apache Software Foundation).