Writing a framework is almost a rite of passage for most PHP developers. Granted, most frameworks don't end up being as comprehensive as the Zend Framework, but that doesn't mean they are not frameworks.
I have personally been through this cycle and I know several other developers that have done the same. You start by realizing that the same basic problems need to be solved in most of the applications you work on. Initially, you copy code to solve the problem from one project to another. Then you start to make enhancements and before you know it, you're working on your own framework.
Eventually, you come to realize that maintaining and optimizing a framework is a lot of ongoing work. Especially when you realize that some of your early design decisions are creating some problems in the long run. At this point, most developers start looking at and using other frameworks. Or better yet, you can start contributing to your framework of choice and have a say in the direction...