> I am baffled as to why you'd build your castle atop a crumbling foundation.

> I have wondered why FB didn't use a proper language with proper typing to begin with.

I would suggest watching Keith Adams's talk, "Taking PHP Seriously": http://www.infoq.com/presentations/php-history

He goes through why Facebook uses PHP and decided to build upon it to create Hack. I highly recommend watching the whole thing, but the main three things he points to are:

1. Frictionless programmer workflow with a short feedback cycle

2. All PHP requests start out with the same consistent state by default

3. Rigid style of concurrency