I am of the opinion that it's simpler to write a basic PHP file than it is to configure Apache, Nginx, Lighttpd. I'd think that the majority of people running WordPress (likely the most distributed PHP project) are doing so on shared hosting, and not likely to want to or know how to configure their own server. The type of person interested in the built-in web server (a decent feature in my opinion, and comparable to `rails s`) is knowledgeable enough to know that the built-in web-server will not cut it for production traffic.