The difference in philosophy is: who is responsible for scheduling work? Is it the language, or is it the developer?

In JS it's the language, for example node sits on top of libuv which is responsible for managing the thread pool and doing async IO.

The advantages of this system are that it's very convenient, and the developer gets a safer single-threaded view over the multiple threads in use. The disadvantage is that the developer lacks control and if you actually want to write multithreaded APIs and not just use them then you have to drop down into a lower level language so you can talk to libuv or the OS.

In C# there is no lower level language to drop down to.