[eluser]John Fuller[/eluser]
Why don't we?
1. Because my needs are different from yours which is why I am building my own from a framework.
2. The community here has been attracted by the framework and not a CMS.
3. A CMS just gets in the way if it is not built for my own purposes (see #1.)
4. If we were to build a CMS then we would just have a crappy general CMS rather than a great framework.
5. If I want a crappy general open source CMS then I would save myself some time and use an existing CMS.
6. I don't have time to work on open source projects.
Interestingly, Zend even takes these ideas further by being more like a "framework store" rather than a fully assembled framework.