You're conflating a couple of things here -- while we don't select for enthusiasm as such, we very much do select (both implicitly and explicitly) for intrinsic motivation. (So the signal of candidates that choose not to apply is not only hard to get, it's also not necessarily interesting.) In terms of where that intrinsic motivation comes from (which is perhaps what you're asking?), we have found that it can be found in a wide variety of places and backgrounds. And indeed, such as there's a common theme at all, it's that it doesn't necessarily follow from a resume.
You touched a very good point - in order to apply, the candidate must be very motivated to apply. Unlike, say, the hundreds of random resumes we (present employer) get every day (feels like a DDoS attack sometimes) - those candidates are motivated enough to fill out a form, but not enough to read the job posting.
