I haven't looked in to it, but just guessing this is probably so that it can do 0-copy sending of files (e.g. doesn't have to buffer/stream the contents through the JVM).
You can also serve files in the more conventional "node.js-style" way (i.e. pump the buffers manually from file to socket) if you like. It's just slower than getting the kernel to do the work for you.
The whole point of sendfile is to make one system call to send all the data in one stream to another, which in general may block. If you're polling and sending only small chunks at a time (whatever you can write without blocking), is it really that much of an advantage over read/write on the same poll? (If you're not doing that, then you have to block, and you have to dedicate a thread to it.)
It's still an advantage over read/write because you're getting the 0-copy behavior.