I find it hard to believe working on the projects they do is the most interesting thing those guys can think of to do for 30+ hours/week. Either they're incredibly boring people or a bit delusional.
I think they continue to work on them because it's mostly pleasant work, they have total control, there's a lot of inertia, and of course because it's making them wealthy.
Perhaps the authors, like many artists, are happy to have the perfect day jobs so that they can pursue interesting hobbies in their spare time. You know, hobbies like blogging, or writing open source software. [1]
There really is a limit to the amount of exciting original thinking that a person can do in a week. You need to sleep. You need to exercise. And you need downtime. If you've got an exciting, fast-moving, forward-looking hobby, it's kind of useful to have a pleasant, renumerative job to do in your off hours.
And it's hard to develop frameworks for building maintainable software if you never actually have to maintain any software. Practice makes perfect.
[1] I cannot believe that I just read a post suggesting that David Heinemeier Hansson might not be doing enough interesting things with his time. How much more code should the guy be giving away? I'm sure there's a non-empty set of people who wish he'd slow down.
I find it hard to believe working on the projects they do is the most interesting thing those guys can think of to do for 30+ hours/week.
I don't.
Sometimes the real fun comes after the business passes its first few milestones. Lots of people bust their butts building a business in order to get it to the point where it will be a lot more fun to run.
I don't know what you mean by "self-sufficient". Neglect a stable company and it will go out of business. The technology market is always changing and you have to keep an eye on your competitors and new technologies.
Sustaining a business over the years and making it through the ups and downs of the economy can be an extremely interesting challenge.
Why are people talking like 37signals is in a sustaining mode? They regularly release new products and significant new features for existing products. I'm pretty sure the people there still get the sense that they're creating something new.
One of the things I've seen is that the business becomes like an extended family -- that the founders get emotionally wrapped up in the people. It's not only the challenge of the work anymore, it's the commitment everyone has made to each other. They stop being at work all the time because they're solving world hunger and start being at work all the time because -- it's home.
Doesn't happen all the time, but I've seen it more than once. I'm sure there are other scenarios that could play out.
Some people may think that doing pleasant work where you have total control and make lots of money is indeed the most interesting thing you can think of. Especially if you have the freedom to tinker with random ideas and enjoy the people you work with.
I am in a similar situation (well, probably not making as much money) and I do find it very interesting. At this point in my life I wouldn't want to lead a company with a limited runway and investors eager to find a buyer for it.
I still get to play and experiment with new technologies, and I use some of our profits to fund at least a couple of interesting projects per year. I could do this for decades.
I don't own the company I work for, so perhaps my outlook is different, but I certainly love my job and find that a lot of the time it's the most interesting thing I do.
I think that some of the "interesting" factor comes in when people rely on your projects. It is the most satisfying thing when you begin a new project with goals, milestones, ambition, and fervor. However, it is very satisfying when people benefit and appreciate what you put effort into.
I don't think technology wise, but they seem to spend a lot of time thinking about and experimenting with user interface and user experience. That is an interesting field no matter what, and satisfying when you get it right.
I think they continue to work on them because it's mostly pleasant work, they have total control, there's a lot of inertia, and of course because it's making them wealthy.