Why doesn't every programmer start a company? It sounds easy enough, but not everyone has the chutzpah (or talent) to start a blog that will support 1+ fulltime salaries.
You can work 20-25 hours a week at a regular job and spend the rest of your time doing your own stuff. It's a reasonably safe setup (if you don't have kids) where you don't give up much but gain a lot, esp. on a personal level. Yes, of course not everybody does that, but I think the argument that your startup (or whatever we call it) won't support you from day 1 is an empty one, if you can make some safe money on the side. I would think journalism is a trade where such a hybrid transitional phase is possible.
