I'm surprised no one mentioned the threading situation in the comments. The main reason why you need a reverse proxy (like nginx) on top of your express app is because node.js is single-threaded. Yes, you can fork (which means a new process and IPC), but that's even more of a performance hog.
If you have to serve a large static file - guess what... no other request gets processed at the same time. It's a neat idea but until we get true multi-threading in Node.js, I'll stick with my nginx, thank you very much.
Node is very happy to asyncronously stream chunks of data over HTTP. You could write a blocking server by collecting the whole response in memory and sending it out all at once, but that’s not required by Node.
This is literally the reason Node even exists, nonblocking I/O with a simple threading model. You can have a million open connections all asynchronously nibbling data as the buffers empty.
If you have to serve a large static file - guess what... no other request gets processed at the same time. It's a neat idea but until we get true multi-threading in Node.js, I'll stick with my nginx, thank you very much.