I get the feeling it's a bit of both reasons. For some, after building and enjoying one, the thought often occurs that if they were to 'do it again' they would do some parts differently and would probably take a shorter time getting to their aim since the brick walls have already been overcome once. Others have a desire to build a different model, maybe a Spyder builder now wants doors and the ability to build on a newer donor such as a Spitfire. Building and selling the first can, and usually will, fund the second pass at a build.
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