It would surely take an inordinate amount of testing, and compiling new and/or additional dependencies for each new version of the many thousands of applications in the software repositories, for the people who look after the repositories to upgrade each application every time a new version comes out.
The stable distributions really contains outdated packages. Just look at Kde, Gnome, Xorg or even the kernel. They are very old. Why is it so?
Well, you might be correct. The age of the packages at stable depends on when the last release was made. Since there is typically over 1 year between releases you might find that stable contains old versions of packages. However, they have been tested in and out. One can confidently say that the packages do not have any known severe bugs, security holes etc., in them. The packages in stable integrate seamlessly with other stable packages.
While many packages in the software repositories are not the latest versions, because there's only so much that any one group of people, who I imagine are mostly volunteers, can do; we can be sure that the versions that are there have all been extensively tested to work on the various operating systems before being added to their respective repositories.
It also seems that the likelihood of an application being upgraded to a newer version, is broadly proportional to its popularity and whether an older version would present a security risk. So obscure command line applications in the software repositories might be several versions behind, but something like Firefox will be updated to the latest version pretty much straight away.
The Debian GNU/Linux FAQ also mentions that with Debian there is a choice of operating systems and their respective repositories; Stable, Testing and Unstable.
(Please note that I have only had Debian installed for a few days, and am not trying to recommend it here, but am just using the FAQ to explain why many applications in the software repositories of most Linux distributions are not the latest versions.)
Stable is rock solid. It does not break and has full security support. But it not might have support for the latest hardware.
Testing has more up-to-date software than Stable, and it breaks less often than Unstable. But when it breaks, it might take a long time for things to get rectified. Sometimes this could be days and it could be months at times. It also does not have permanent security support.
Unstable has the latest software and changes a lot. Consequently, it can break at any point. However, fixes get rectified in many occasions in a couple of days and it always has the latest releases of software packaged for Debian.
The same is true to a lesser extent and in a slightly different way, with Linux Mint, as it holds back certain updates by default.
Edited by Al1000, 20 February 2015 - 06:33 AM.