> BufferedReader.read() calls FileIO.read() until FileIO.read() returns
> an empty byte string. Why not calling FileIO.read() only once?

BufferedReader doesn't call FileIO.read, it calls <rawio>.read().
The latter can be e.g. a socket and call recv(). If you want to read
till the end of stream, you have to repeat until recv() returns the
empty string.