Adapted to your needsA framework, per definition, is generic. If you develop one yourself and you do it right, your home-made framework will end up having as much abstraction as a public one.
On the other hand, a public framework can be adapted to some specific needs you may have with less effort than developing one from scratch.
What about tests ?A public framework usually comes with a full stack of unit tests. And every modification or addition to the code will have it's accompanying tests.
It will take you a good deal of time and consistency to accompany each and every change to your home-made framework with it's unit tests.
DocumentationYou're developing a framework for your company so everyone can take benefit of it. So you have to document it properly. Not just API documentation, but also a developer reference guide so anyone new to the company and the framework can get up to par swiftly. That requires time too. Writing documentation is not that easy as you might think it is, either.
Public frameworks -at least the major ones- come fully documented and the documentation is being kept up-to-date as the framework evolves.
SecurityThe framework you're developing will be used by not that many people (unless you release it out in the open and it becomes a success, but then I guess it'll have no specificities to your company any more) and security flaws and bugs will not be detected rapidly.
Public frameworks are being used in many applications. So security is one of the major concerns of the contributors of a public framework. Besides that, many people using the framework in many different ways, bugs are rapidly detected - and corrected.
SupportBy it's nature, the proprietary framework will have a far less big user base than a public one. So support for it is naturally reduced to a few people, if it is not just one person.
I'm quite sure you'll often get a response to a question more rapidly and accurately from the user community of a public framework than you will from the gurus who wrote the one for your company.
RecruitmentLast but not least, it'll be easier to recruit if you're using a public framework: it's quite obvious no one will know yours!
ConclusionIn the end, and in almost any case, there's no valid reason for your company to develop it's own framework: it'll cost more time & effort than adapting an existing one, and that includes performance optimization if needs be.
As a side note though, I'd encourage any developer to try and build his own framework or actively contribute to an existing one. You learn a lot from it!