They may not encourage transfers because the FAs have been hired because they posess a certain set of skills and the flight crew have been hired becuase they posess a different set of skills. If a person is hired for a job in the cabin they may be ideal for what the company needs in the cabin but the total opposite of what they are seeking personality-wise in flight crew. There is significantly more to becoming an airline pilot than just collecting a clutch of licences and ratings. That said I know of three people who have made that transition succesfully, so it's by no means out of the question.