I do not get paid every time it runs for the rest of my life, so why should you?

Sorry if I misunderstood you, but this feels rather easy to answer: because you are being paid to write the code. Spotify doesn’t pay anyone to write music (well maybe they technically do for some ads or something, but it’s definitely not how they acquire more music to add to the library), they just pay for streaming rights on music that was somehow already independently produced. And tiny unknown musicians have no leverage to negotiate better terms than what Spotify offers.