Their last mainline title, Grand Theft Auto V, made $1 billion in three days [0], and much, much more over its lifetime, even ignoring its extremely profitable multiplayer mode (GTA Online).
You asked why would people work this hard over a "video game". Presumably, you don't work at Rockstar and aren't in a position to gain financially from them, but I don't think it should be too hard to understand how people could be convinced (right or wrong) to do a few "crunch" weeks when the deliverable has a hard deadline and huge revenue potential.
