Why are so many of the class declarations implemented as final?

I'd love to implement a version of the ProjectFactory as an example, that adds some middleware to the new project definition, but right now I can't. I can implement the interface, sure, but then I'd have to re-implement the entirety of the namespace logic to get a Namespace tree. I can't even wrap the class because the namespace processing is all protected.

Similarly, I'd like to use slightly modified Factory\Function_ and Factory\Method strategies, but would need to copy pasta much of the code because again they are final classes. Being able to extend these classes would be a huge benefit and reduce the amount of code I'd need to write considerably. Perhaps I'm missing something, but they seem constructed with sensible defaults and are itching to be extended for additional implementations. Any reason they are not?