Because Java distributes applications as bundles including the libraries (fat JARs, WAR files and so on), libraries need to be available to the builder, not the installer. That means it's perfectly sound that they're in Maven Central (and competitors), and not in the OS's package manager. It's the same with packages in JavaScript, Ruby, Go, Erlang, etc.
The only things you might need to install through OS packages are the JVM, and perhaps an application server if you're doing things that way, which fewer and fewer people are.
When i first started working with Java on unix, i felt the same way as you - i wanted the libraries to come through the OS package manager, the same way native libraries do, and spent ages trying to get my deployed applications to use them. Eventually i realised i was just doing it completely wrong.
The only things you might need to install through OS packages are the JVM, and perhaps an application server if you're doing things that way, which fewer and fewer people are.
When i first started working with Java on unix, i felt the same way as you - i wanted the libraries to come through the OS package manager, the same way native libraries do, and spent ages trying to get my deployed applications to use them. Eventually i realised i was just doing it completely wrong.