Because he wants to have tons of money in the bank and get up every day to go do anything he wants with his four-year-old kid when he wakes up. Building something in a couple months that got popular enough to be able to this is amazing. Not check GitHub issues and PRs from ClawdBot5000 and hope he'll make some money from it in 5 years.
We're entering a new world of software development that allows people with no coding experience to do things like build their own mobile apps, websites, and all sorts of other things that were previously reserved for only technical people. The gap is closing quickly. At this point, if you're a "real" developer and have an app that somebody wants to purchase, you'd be crazy not to take the offer if it's a good amount and have cash in the bank that would allow you to do your own stuff in the future without worrying about $dayjob.