tl;dr We use Symfony.
When we work in a company, we can't afford to create projects on different framework. Every developers need to be able to quickly work on a project he/she doesn't know. We need to ease their work by not adding another complexity such as learning a new framework.
Of course, as PHP developers we should be able to work on any framework but still, it does not mean we can be as
efficient as when we work on a framework we already know.
Moreover, it is very difficult to recruit good developers in one framework so asking applicants to know every single
frameworks available in the market is not possible.
Still, the PHP community evolve and we are not closed to new frameworks. But this choice must be carefully thought.
There should be a lot of advantages for a new framework to be chosen over the default one.
Symfony is one of the big framework. Lots of its components are reused by some well known library (the yaml component is
used by PHPUnit) and even other frameworks (Laravel uses a lot of symfony components).
It has a large community who produces lots of bundle and most of its design choices are inspired by frameworks in other
languages.
If you work with us, it probably means that you are already convince that Symfony is a good choice.