Moreover, they know they were directly working on a game that made at least (1.5M * 39.99) ~60 million USD. I admire dedication to one's work and love of the game and all, but that's just not rational.
It doesn't change your broader point, but I suspect that could be a significant over-estimate. Steam take ~30%, and many copies was have been sold at a discount price (I paid ~$15) due to the frequency of Steam sales.
In open source you keep the rights. Which for this game probably means millions. They did not get the rights or Rúe money. It just baffles me to no end.
It's entirely likely they didn't know that - apparently none of the people working on it were told how much it sold and the company tried to stop outsiders estimating its sales figures through services like SteamSpy.