eric5h5: It's still easily enough that everyone involved was making decent money. (And I doubt there's renting office space or stuff like that since it's highly likely this is several people working together remotely, which is typical.) Again, this was never about "have to get a full-time job" or anything. Just time to do something else.
there are also economic security. Barth have previously stated that the job he has now is his first full time employment, and that he have had it for a year, but found it difficult to do full time game development and full time work. yes, he might enjoy game development more, but is he married? does he have kids? do he need to think about college funds? making sure the mortgage is paied each month? having a pension when retiering?
Game development is quite fickle, and you more or less live from game to game if you are a full time indie developer, which is why most have a full/part time job and do games on the side. if you have commitments, then i can understand why you would go for the more secure (though pershaps the more 'boring') option
§pec†re: There's also PR and advertising costs to get the attention on the games to start out.
to be fair, Zachtronics never have much PR / Advertisment. They had a core group of very lojal fans (/wave) and the news of the games spread by word of mouth. when they got 'famous' for the games they made, then the game press laos made interviews and reviews od the games as well, which Zachtronis did not need to pay for. I do not remember seeing any specific advert for any of their games, apart from the teasers they made themselves for their own website, which were also used everywhere else as well.