This isn't the first time I've read about a top employer asking a highly-desired employee why they'd never applied in the past, and receiving as an answer, "oh,

This isn't the first time I've read about a top employer asking a highly-desired employee why they'd never applied in the past, and receiving as an answer, "oh, I didn't think I'd make it through your interview process" Exactly. And to someone who is already gainfully employed elsewhere, the opportunity cost of going through an interview gauntlet--which often consists of one or more full days of intense testing, and many more days of practice to prepare for that testing--is very real and daunting. So they will naturally tend to attract fresh college graduates who have nothing to lose and can afford to make such a huge time investment in the interview process for a particular prospective employer.