He mentioned in the comments that the reason why he has not made a multiplayer game is because he is only a so-so programmer and lacks the skills to do so.

You're making it into an economic question when it isn't one to the author. He likes making games like this, so this is what he's decided to do.

I bought one of the games recently and emailed him a bunch of usability/interface suggestions. He thanked me but mentioned that he's making a living so somebody must be enjoying the games. He's got a point.

It's a labor of love. I'm sure there are a million things he could do to add the potential to make more money (like an investment in better art or another programmer), but it looks like he's not interested in that kind of risk.

It doesn't seem like an ambitious plan, but he seems happy so what's wrong with not trying to experiment with business models and whatnot?

You're forgetting, he has many other games already out that bring in recurring revenue. Even if he's taking a 60k salary while building this game it doesn't mean he's not making much more from the other recurring income.