I have not found this to be the case. There are a lot of small/medium sized companies that probably started out with a custom Drupal/Wordpress site and have now out-grown it, but don't want to re-write the whole system just so that they can use Python/Ruby.
Using PHP is then a no-brainer and so moving the codebase to Symfony2 is a great option for them. While Wordpress and Drupal command such a large share of the website market, there will always be decent young companies emerging who are also tied-in to the PHP community.
I also had a look through the Symfony2 docs. Perhaps it's just me being a Python bigot, but the code there doesn't look any better than the raw PHP: http://symfony.com/doc/current/book/from_flat_php_to_symfony... And the "helper function to render templates" in the section above it also looks fairly deranged. Isn't that included in the framework somewhere?
The bits that do look ok seem to be because they're copying Django/Jinja2 templating.
If you stay on the same web stack you might be able to migrate one piece at a time.
You also have the option of refactoring the most important or troublesome bits of your app to use Symfony2 while leaving the rest alone.
It's pretty challenging to take one piece of your big web application and migrate that to Python while leaving the rest in PHP. So you'll tend to do it all at once. That's very expensive, and very risky, even if your original ugly PHP site had an extensive test suite, which it will not.
A framework shouldn't (and most all the popular PHP frameworks out there, including Symonfy adhere to this) matter when your business logic is decoupled from from your framework. Yes, you might have to rewrite some constructors. And some frameworks have different features, but can still be carried over if you need it. You aren't beholden to a single code base.
For example, I can easily move my models from Zend to Symfony without rewriting a thing. Decoupling is a wonderful thing.
So, you have this code base you are using, these libraries that are already written, and tested, and working. Moving to Ruby/Python would require that they all be rewritten, and appropriate libraries be replaces.
Moving to another language isn't a weekend endeavor when you have a workflow and toolset built up over a 10+ years of development in another language that, in that domain, accomplishes the same thing.
Using PHP is then a no-brainer and so moving the codebase to Symfony2 is a great option for them. While Wordpress and Drupal command such a large share of the website market, there will always be decent young companies emerging who are also tied-in to the PHP community.