Because apt-get doesn’t exist on the system I use, and most system packagers can’t/won’t keep up with the rate of change of development, anyway.
CPAN, RubyGems, etc. all exist because system packaging systems are (a) specific to a particular distribution, (b) generally many versions behind the development versions, (c) really hard to get into unless you are already a system packager for a particular distribution, and (d) effectively don’t exist for two of the largest platforms where you will find users.
I’d suggest making some sort of meta-system packaging system that works for all the languages, but xkcd 927 (and I wouldn’t use it myself, anyway, because it would probably not install Ruby packages correctly and would solve things worse than NPM already does).
CPAN, RubyGems, etc. all exist because system packaging systems are (a) specific to a particular distribution, (b) generally many versions behind the development versions, (c) really hard to get into unless you are already a system packager for a particular distribution, and (d) effectively don’t exist for two of the largest platforms where you will find users.
I’d suggest making some sort of meta-system packaging system that works for all the languages, but xkcd 927 (and I wouldn’t use it myself, anyway, because it would probably not install Ruby packages correctly and would solve things worse than NPM already does).