Where are you? I believe my experience is typical of large tech companies in California, which was the question asked. I'd be shocked if a former contractor got hired without at least a half-baked pretense of an interview, if only for legal reasons--to treat contractors differently from other non-employees risks creating a presumption that they're not non-employees, which HR built this whole elaborate mechanism to fight.

And the agencies are absolutely fine with it. Sometimes there's a buyout fee, but not a big deal. The agencies are pretty interchangeable anyways, so they can't be too picky.