Why Facebook, Google move to statically typed language?

Why Facebook (hack), Google (go) moving towards statically typed languages? Is it true that it is difficult to maintain a large code base of a dynamically typed language?

Facebook: https://code.facebook.com/posts/264544830379293/hack-a-new-programming-language-for-hhvm/

I could not find an article from official Google source, but I heard from one of its employee that they are switching for most projects from python to go lang.