Actually there are lots of pilots that switched from GA to airlines, airline pilots that fly freelance in their free time. However it is true that there seems to be a much higher amount of "cowboy" pilots and companies in the GA. Some of that probably has to do with the fact that most airlines do select their pilots very carefully and regulate them and their work quite a lot more than is usual in the GA. Another part is external auditing, for example IOSA audits.
The results are quite obvious, a much lower accident rate in airlines than in the GA in general, of course there are some on both sides that do much better or worse than others in the same business in regards to safety.