1. Developing multilayer games requires more work than developing single player games. You have to look at the cost/profit ratios, not just the profit.
2. The work can be qualitatively different - multilayer games are not designed the same way as single payer ones, you have a whole new can of worms to worry about.
3. Maybe he likes single player games (or making them) better than multilayer ones.
as a side note - 120k usd is not that much money, considering most of that money is the author's salary.
Considering the other expenses in that budget, 120k gross expenditures sounds more like a 60k salary for him.
Obviously it's trickier to develop a multiplayer game, but the economics seem so compelling; in a 15 year career, he's never broken out into multiplayer? Or is there something I'm missing about those economics?
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.
2. The work can be qualitatively different - multilayer games are not designed the same way as single payer ones, you have a whole new can of worms to worry about.
3. Maybe he likes single player games (or making them) better than multilayer ones.
as a side note - 120k usd is not that much money, considering most of that money is the author's salary.