As a writer I feel like lots of commenters on here are missing something important. Caveat, I know Julian slightly. We're not friends, but the writing world really is that small.
This is not about how many hours he's speculated to have spent on the piece, or his overall contribution to the success of the game. It's clear that Minecraft was already phenomenally successful by the time the end text was written. It's also clear from other comments here that a) Julian was far from the only person who didn't get a 'share of the winnings', and that b) the person responsible for the music in the original version(s) of the game C418, by contrast did (likely due to having much more business savvy).
Two points...
1) None of this is fair. Not how writing is paid for. Not how writing software is valued commercially over writing / scriptwriting / poetry. Not the Minecraft team making vast sums. Certainly not Notch making billions. It's all an absurd outgrowth of the economics of digital surplus, which are now so ubiquitous as to appear invisible. And network effects, which all of us on HN are aware of, but which are also deeply unintuitive. Is Minecraft a great, influential, fun game? Sure. It is so much greater than the other successful and brilliant games of its generation of indie titles - Fez, Super Meat Boy, Braid etc? Frankly no. It's just different. It's different in a specific way thats amenable to economic exploitation indefinitely. It's more like a toy than a game. It came out at a very specific time in the history of videogame culture and technology when something less graphically impressive could be appreciated. Initially by a generation who had come to find an appeal in retro graphics, before it caught fire with the kids. It was released at a time when the community and internet ubiquity existed for such a thing to go viral. Great things are created and ignored all the time. Minecraft hit at exactly the right moment. There are numerous other examples in the gaming space of titles that aren't necessarily so unique (Minecraft is extremely derivative of Infiniminer). A world exists - very close to this one, in which Zachtronics would have a legal right to some or all of its profits, had patent law shaken out even slightly differently.
2) Writing (like music) is non fungible. The story you write in 2013 is not the story you will or could write in 2022. It expresses a part of you, in a profoundly intimate way. Publishing is giving away something of yourself. It's irreducible to sharing an idea or emotion - because it's more than that. All creative writing is literally an expression of the self of the artist. Publishing is also a one strike deal usually. You can't often publish a story that is already public, virtually no short stories (or indeed novels) have a (commercial) value in adaptation. It's starkly different from say music - where you can tour your work, even if Spotify pays effectively nothing. You can't busk writing (well you can, but there's no audience for it beyond a novelty). Most commercial novelists (even ones you've heard of) subsist from teaching workshops and art grants. Most have second and third jobs. Which is all to say that this poem was a valuable part of Julian that he shared with the world. He tries to articulate it by the muse or automatic writing. Writing not equivalent to time spent coding or problem solving. It's an act of love. As is his sharing the final story for free in perpetuity.