I think the origin of this idea comes from the time when personal computers started becoming available, and a generation of then young people started tinkering

I think the origin of this idea comes from the time when personal computers started becoming available, and a generation of then young people started tinkering with them. Some of us then went on to do some version of this hobby/passion professionally. In that case, having software side projects doesn't seem so weird. Analogies that come to mind are folks like climbing instructors and mechanics, who also may have had these activities as fun hobbies before making a living at them. Nowadays there is a higher percentage of folks who got into software (or hardware or whatever) the same way the bankers, lawyers, and doctors did; it was presented as a good career choice if they learned it. I agree that expecting those folks to have side projects is unreasonable.