Selling a Script Built on a PHP Framework
It really seems that a lot of people have missed the true nature of the question and even taken it as far as language debates (those never end well).
Is there any reason why I couldn't resell a script built on one of these frameworks as a full-blown turn-key solution (especially if the framework's licensing is something very flexible, like the BSD license)?
Assuming the framework license permits it then there's no reason you couldn't do this. You had mentioned Zend Framework so you may be interested in looking at Magento. While they offer a free community edition they also have a paid edition that works with the Zend Framework as well.
I recently worked with a file upload script that was offered commercially and it happened to be built on codeigniter (name escapes me at the moment).
If not, why aren't others doing the same thing?
My personal opinion is that it's a mix of quite a few factors really. The web based market for on premise applications (as apposed to SaaS) is already flooded with options and is starting to shrink in size. This makes less demand for an application that you would actually see the framework behind (with SaaS you most likely will never know what framework if any is being used).
A lot of the existing large players in the PHP market have been around for a while and already have their own code base that they have created and are familiar with. When you've spent years building your own libraries it's hard to justify moving to another framework.
A lot of the smaller players rarely educate themselves in proper application design and usually stick to procedural code. The big OOP features that exist in PHP today didn't come along until the 5.0 release. Mind you that was around 5 years ago but a lot of your programmers had started on their PHP tutorials and learning adventures before PHP5 was widely available and accepted on standard hosting accounts. As such most of our modern frameworks were not available CakePHP as an example didn't start until 2005. Zend framework wasn't around until 2007. These are all relatively new dates and I wouldn't expect to see a lot of commercial applications moving to them until the current generation of programmers that can write quality commercial applications age a bit (again just my opinion).
I have to heartily disagree with back2dos..
PHP's a solid, incredibly well used programming language for developing web apps. It can, of course, be used for commercial development and millions of people (me included) do just that. I'm not sure PHP bashing is really relevant here.
True, PHP is not compiled but if you really care about this you can use Zend Guard which can encrypt code. Personally I've always found open source code a plus point. Clients like to know they can get at the code if they really need to, it offers some reassurance.
There are lots of OS PHP apps, some great, some awful. Find a niche (like any business), something that has real demand, and develop for that.
So I think you're fine to develop commercial apps/scripts. Just make sure you give them decent support and documentation. You'll find people appreciate that and are willing to pay for it.
Finally on the point of your question, I agree they stand a much better chance of being used if they are based on an open source framework since you'll be opening yourself up to wider market. Zend Framework, as you may know, has a pretty open license which says you can sell anything you develop with it.