Probably because it's not a very interesting problem to solve, there are MANY adequate solutions out there that all suck in their own special ways, but are good enough to solve the actual problem at hand, enabling people to move onto working on solving more interesting problems.
Here’s a site that catalogs CMS systems and software. 1300 and counting. I’m reasonably sure everyone thinks they’re building a good one.<p><a href="http://www.cmsmatrix.org/" rel="nofollow">http://www.cmsmatrix.org/</a>