Somebody I spoke to recently had a nice analogy for it, which I've forgotten. But I think it's more to do with a failure to balance the fun stuff with the nitty gritty, the value-add that that brings in your daily bread. A writer, for example, would be crazy to think they can make a living just writing about their travels and selling it to the national newspapers. It's just not enough on its own. But if they start out with a more realistic outlook, then it's no problem. If they are willing to do some less glamorous activities, which are well paid, then they can enjoy the best of both worlds and make a good living.
One of the most appealing aspects of being self-employed is that you are in control of your own time. There is so much abundance there, and so much potential. Why limit yourself to one specialism?
Which takes me to my final point - time management. 'Nuff said!