Historically speaking, Arc was put forward as a language for experimentation. If you want to package up code in a squeaky clean format for others to use who don't care about implementation details, you're doing something other than experimentation; you're publishing a product people would supposedly rely on.
Moreover. not even the Arc language is stable (or so this website has always said). If it encouraged people to package up their stable code, that would prove deceptive once the whole foundation fell away. This happened to Anarki when Arc 3 was released; the community had built up lots of Arc 2 code, and not much of that was ported to work on Arc 3.
However, Arc is not without module systems. The first thing I did in Arc was build some namespace macros with explicit imports and exports for my own benefit (Lathe). Andrew Wilcox made "the hackinator" which I think could download Arc programs from a Web-hosted manifest file, most often from GitHub.
More recently, Arc/Nu is an Arc implementation with rich support for first-class namespaces, along with several utilities for modular loading of Arc code that's written in the usual sequence-of-commands style. Arc/Nu is probably the best platform to publish stable modules for, since it can load alternate namespaces to achieve backwards compatibility.
