Why do people choose to start a project with PHP? Ever.

I If you were going to create any type of serious program for the web, wouldn’t it make more sense to use a more robust language like Java or a .NET language? Yet, very large, very successful web applications are built on PHP (Wikipedia, Digg, Facebook, the list goes on…). New and innovative web applications are constantly being developed in PHP. Entire web development companies code in nothing but PHP! WHY!?