They do this not because they are forced to, but because it cleanly addresses a problem they were having. FHS 2.1 should be available by the time this article is published. The widespread availability of third-party commercial software for Linux is a relatively new phenomenon. Many vendors are faced with a problem they have never seen before. At one time, all the software on your Linux box generally came from one of two places.
Distributions are very good at integrating software and making sure everything works together.
Assuming someone had the expertise, it is possible to get even a sub-standard program compiled and installed for your Linux boxes. They are forced to consider which distribution you run, what version of the distribution, your kernel version, and a slew of other considerations before they can be reasonably certain that the application will install correctly on your system and work. If the goal is to provide a binary package for everyone, it is even worse. You must tailor the application for any and all distributions, and then compile, build and test it on each one, too. One of the more common fears about Linux is that fragmentation will occur because it uses an open-development model.