Sometimes having a great product really is enough.
I do think, though, that even low-sophistication one-man-shop businesses are substantially more complex than "focus 100% on product and everything turns out cool."
Don't worry, I'm not saying this to drum up consulting business. All of my clients are waaaaay past the point where what I said here would be controversial. Many of them have CEOs who describe their own jobs as figuring out ways to protect the engineering team from the other 90%.
You've committed a fallacy by looking only at the successes and determining it isn't difficult. The vast majority of businesses fail, and you never see them. Most of the rest never amount to anything more than someone busting their ass for the same (or less) money than they could be making punching the clock 9 to 5. That's the fundamental truth of it.
But of course, we live in a world where everyone is above average, right?