Why is Node.js single threaded?

In PHP (or Java/ASP.NET/Ruby) based webservers every client request is instantiated on a new thread. But in Node.js all the clients run on the same thread (they can even share the same variables!) I understand that I/O operations are event based so they don't block the main thread loop.

What I don't understand is WHY the author of Node chose it to be single threaded? It makes things difficult. For example I can't run a CPU intensive function because it blocks the main thread (and new client requests are blocked) so I need to spawn a process (which means I need to create a separate JavaScript file and execute another node process on it). However in PHP cpu intensive tasks do not block other clients because as I mentioned each client is on a different thread. What are its advantages compared to multi-threaded web servers?