With Plone, you can see what is possible...with Zope. With Zope, you can see what is possible with Python.
The problem here is that Plone is not a language; it is a CMS. And Zope is not a langauge; it is a framework that happens to be written in the language Python. Python otherwise shares the same problem as PHP: when you start a new project, you either have to find some existing code or reinvent the wheel. Zope minimizes this by providing a framework in which you can work and find a lot of the code you already need -- the wheel is already there.
There are many, many PHP frameworks out there that try to fill this need. The trouble is that there isn't a single PHP framework that has achieved the same level of notoriety as Zope. However, more and more people are using PEAR [pear.php.net] as a framework, and the beauty of PEAR is that you don't have to use the entire framework in one installation; you can pick and choose.
So, people in the PHP community are working to solve the wheel reinventing problem; it's just that many people either don't know about it or don't want to learn to use the existing frameworks.