It's because people are too obsessed with providing complete instructions to incorporate any package manager into their instructions.

What we are really missing is an explicit progression from new software to maintained packages across distribution. As it is, each distro expects each package to have a maintainer, and very few people actually want to do that across several distros just to release their software. Generally, the expectation is to instead just wait around for people to make and maintain those packages by virtue of their own interest in your software, but it takes a while, and discoverability isn't automatic.