It is unfortunate that a game as famous as Dwarf Fortress only makes so little in money: http://www.bay12forums.com/smf/index.php?topic=152358.0
Found it: they were offered 300K just to license the name.
He has refused a programming job at a major developer
(he asked that I keep its name off the record) and
turned down a $300,000 offer from another company to
license the Dwarf Fortress name, fearing that the
proposed sum wouldn’t sufficiently offset the long-term
donations drop that would likely result.
They aren't, though, so they're making a conscious decision to forgo that money in order to keep their vision (as maddeningly abstruse as it is) intact. In that respect Dwarf Fortress is a lot more like an art project than a commercial game.
For a comparison, Gnomoria (http://gnomoria.com), which is an unapologetic DF clone but with basic graphics and first-party modding support, sells for $8 a pop on Steam and has 2,000 reviews despite still being in "Early Access" status.
I think it's best to think of Toady like a crazy man building a cathedral down the road, single-handedly, using only tools he made himself. We can only sit and watch in awe and appreciation as the cathedral takes shape. And support him on Patreon, I suppose. :)
$30k per year each is still pretty good tbh
Per capita median personal income in the USA is right around that level, so 50% of the population earns less, while being statistically unlikely to love their job. If half the country hates their lower paying job, they have little to complain about in a relative sense.