I think its because so few people get paid to wrote Lisp.
https://www.patreon.com/sindresorhus
Most JS devs know sindresorhus by reputation. You can't help but knowing him through his work. He even put up a separate twitter solely because people kept wanting a way to see whenever he made a new github repo.
Patreon makes him $3300/mo. So that appears to be the ceiling for all of us.
Hopefully it becomes an average, not a ceiling. We're not all going to make the next Vue, but that guy is getting $16k/mo https://www.patreon.com/evanyou .. that's what I consider the ceiling.
And closed source isn't either when it comes to productivity tools (because there are so many free alternatives).