Why not? There's no reason in principle that you can't have hot standbys that are switched over to immediately when the primary fails. Or even a no-primary setup with each cluster being master for some data and slave for others (a la Cassandra's replication model).

There may be specific aspects of Github's usecase that make this difficult, but please don't pretend that geo-redundancy is impossible. Look at Netflix's architecture for an example of a site that services traffic from multiple AZs.