I'm a freelancer by profession - it's a perfectly valid form of work. It just needs someone to understand more than just the skills they're being hired for. Being a ghostwriter also means knowing how to negotiate rates, how to adjust client expectations around outlines, how to pitch according to a client's needs (some clients may want someone who can recreate their vision with the most accuracy, some want someone who'll create market-ready work so they can recoup their investment, and your pitch has to reflect that), how to communicate progress, how to make clear and achievable promises, how to network (freelancers get a lot of work by getting clients to bring them their friends), how to "stack" projects so that you'll have future work when you've completed your current project, etc. It's a lot of work to do it right, but that goes for any profession.