Other alternative question: How Satoshi makes money by create Bitcoin, Bitcoin Core and this forum? the answer is not every person that invent something is purely for making money.
Pretty bad example, though. Satoshi
invented money, he didn't just make an SPV wallet. Sure, he did risk his time and energy, but in a way, he did pay himself.
Another noteworthy detail, is that people working on free software around Bitcoin, completely voluntarily and with their hours of operation, are likely retired already. I wouldn't be surprised if OG Bitcoin Core developers or ThomasV (as an example of an old SPV dev) sit on top of big stash and have decided to maintain the ecosystem for the rest of their lives.