It's one of the few highly paid fields where you can do it for fun. You cannot really practice medicine or law in your free time on whatever you want. You can only really do charity work in your free time. Not medical or legal research or anything like what OSS / personal projects are for us.

For other types of engineering, I think there is a bit of a thread of doing it for fun. The Homebrew Robotics Club comes to mind. The easier it is to actually create something in your field, the more there will be a thread of doing it because you love it.