If their software was all available and open source, I could host their software and my repositories on an $11/month Rackspace virtual machine, and still have about 7 gigabytes left over (or more realistically just stick it on a vhost on some machine I've already got on the internet and host it essentially for free).
This is the brilliance of Github. They have found a way to sell storage at several orders of magnitude over the going rate--all because they have coupled it with some attractive proprietary software. Their value is entirely in that proprietary software.