Well, probably because it doesn't work. In our experience, the overwhelming majority of potential employees either have jobs already (and so don't have time to

Well, probably because it doesn't work. In our experience, the overwhelming majority of potential employees either have jobs already (and so don't have time to work on contact for you while you determine fit), are getting a job soon (and understandably don't want to turn down a full time position so they can contact for you and maybe get a job later (or not)), or just are offended that you won't commit to them and wasn't too try before you buy (which may be silly, but I admit I felt exactly that way when someone wanted me to do contract to hire).