Because elitest attitudes garner idiotic responses and I'm sick of them.
Yes, Facebook had to write their own interpreter, but you also know what? Facebook also actively helps the PHP community and is ushering the community too. So anything Facebook creates they open source and PHP7 has taken cues from them. The whole net benefits from it.
Not to mention the next biggest website that is powered by PHP is Wikipedia, whom uses stock PHP to serve their sites.
Facebook has so much interactions and that, that they warrant the need for what they did. 99.9999% of most people don't need that. Not even Wikipedia. They just render a html page. Done.