Ask HN: Why do distros ship old software?

Why do the most popular Linux distros ship old software? Some distros will even go so far as to create their own custom patches to fix bugs in older versions of software instead of just pushing out the latest version. I can understand when it's the kernel and they want to avoid upgrading too often, but why do they do this for packages like Apache, or MySQL, or Ruby? Why do they call those old versions stable? Surely the latest release from the developers would be more "stable" than a 2-year-old version with some backported security fixes.