OS X/Darwin follows the design philosophy of it's cousin FreeBSD. The base system is only upgraded when a new OS version is released. Upgrades may or may not be the latest version of a software package. If the tool does it job and does it well then it may not be upgraded. The idea is " the lastest version may not equate to a better version" or "if it ain't broke, then don't fix it". So, the base system on your version of OS X will only be patched when there is a security concern or severe bug.