Sure, Github can lose data. And you can lose data. But the advantage is that you and Github are much less correlated; the odds that both of you will lose the data at the same time are fairly low. 
Data safety is all about fighting correlation. You don't back up one partition to another on the same spindle, because when the drive dies the whole spindle is lost. Paranoid people back up to two different drives, two different disk controllers, two different machines, two different datacenters, two different continents...
 But nonzero. It is worth thinking about the scenarios.
Agree 100%. Furthermore, it's important not to conflate version control with data backup. Although they share some traits, they have different goals. For example, if I lose my local working copy of a repository before any commits, I've lost valuable work. In absence of a good backup strategy, the existence of the remote repository is of little consolation.