That's not a hard dependency, it's a sensible default that you should be overriding if you're running composer install as part of your deployment process instead of including composer's vendor directory in your repo. Documentation: https://getcomposer.org/doc/05-repositories.md
Practically all packages use git sources. Yes, you can vendor them - congratulations, you just kicked the can down the road and moved the issue to the build step. The issue exists for other languages as well - maven/mavencentral, ruby/rubygems.org - but only composer depends on github very much. I don't like that but it's not like the other solutions are much better.
