>How many people in other industries voluntarily embark on pet projects or hobbyist experiments that involve the same things they do at work?

Well this behavior isn't limited to those involved with software development. I'd say that this is the case for any profession that involves the formulation and then creation of content, ideas, products etc.

Take this guy for example: http://www.slate.com/articles/arts/culturebox/2012/01/the_be...

A lot of other fields necessitate a customer in order to perform, such as medicine.