The purely mathematical reason is that there are easily more private jobs available than academic jobs.
I think it's unnecessarily pejorative to say most candidates flat out lie. Keep in mind that virtually all of the rad onc residency training programs are at academic centers, and applicants understand that academicians like to be told how great academic medicine is. It shouldn't be difficult for an applicant to stress aspects of his/her application which are in keeping with an academic mission, even if they're 97% certain they're going to do private practice. This isn't "flat out lying", but it is playing the game to a certain extent.
And yes, some truly intend to do academics and have a change of heart. I've actually gone the opposite way, and am starting to give more consideration to academics. But the underlying reality behind you're question is simply one of numbers.