Instructors are indeed very employable... just look up "McDonalds" in the phone book...
Seriously though, few charter companies need more than one or two check and training guys, and virtually all their pilots have probably instructed at one time or another... besides, the charter companies know that most of their new hires are just using them as a stepping stone to something else, so why invest the money in training them up?
Charter and instructing are two different things, being good at one doesn't make you good at the other. You think in different ways, and apply different criteria. For example, very few instructors have any experience in bad weather, because they don't fly when the wx is crappy. Same goes for experience flying an iced-up twin with marginal instruments and cranky nav kit. And then there are real-world command skills, like knowing when to divert and when to press on... few instructors do that for real with any regularity. And what squawks you can carry safely, etc etc etc.
Not that it can't be learned quite quickly... but employers like you to have it at the start.