I don't work there and don't know anybody who works there, but I do know that they have been using proprietary dialects of PHP for some time. React's JSX was in

I don't work there and don't know anybody who works there, but I do know that they have been using proprietary dialects of PHP for some time. React's JSX was influenced by XHP, and XHP was released in 2010 but apparently was in use for years before then. So from an outsider's perspective, it seems like Facebook has been using proprietary extensions or entirely proprietary languages for over a decade. The fact that Facebook is such a big company means that they can afford to have entire teams working on these proprietary languages, much in the same way Google had the ability to assign full-time engineers to maintaining their internal container infrastructure (Borg, the predecessor to Kubernetes). Because of this, I don't see why they would give up control of some of their stack to people outside their company, who may not share their values. This isn't exclusive to Facebook's PHP developers...they have their own libraries for C/C++, Swift, JavaScript, and much more. It's a big company, the developer tools are going to be a lot bigger than most other workplaces. Again, this is pure speculation. But most of these big companies do the same or a similar thing. Both Apple and Google have invented their own languages, the difference is that Facebook doesn't have an app platform that requires you to use Hack/PHP, thus nobody is going to just pick it up for no reason unless it compels them in other ways (e.g. Go)