Not only is that not true, it's demonstrably not true: when people write code, they actively want other people to use it. By way of example, the author of the library we're discussing here, "Monocypher", has declared it "ready for production" and has a web page selling its virtues versus libsodium and NaCl.
People who write crypto code as a rule are not doing it for their own edification, which is why so many more people spend time writing libraries and encryption tools and so few people spend time writing the code to exploit crypto vulnerabilities.
You said that it's almost certainly the case that people building their own crypto are doing it solely for their own purposes and that people won't even bother to complete their work. Obviously: no.
I wrote "almost certainly the same is true of crypto" meaning that almost certainly very few people actually want to implement crypto (just as very few people actually want to build airplanes) for any reason, and so the desire to do so serves as a pretty effective filter to eliminate "random people" doing it.
Of those few who want to do it, some will want to do it for personal reasons and some will want to do it for commercial reasons, just as in aviation.
People who write crypto code as a rule are not doing it for their own edification, which is why so many more people spend time writing libraries and encryption tools and so few people spend time writing the code to exploit crypto vulnerabilities.