Why does ENOENT mean “No such file or directory”?
It's an abbreviation(縮寫) of Error NO ENTry (or Error NO ENTity), and can actually be used for more than files/directories.
It's simply “No such directory entry”. Since directory entries can be directories or files (or symlinks, or sockets, or pipes, or devices), the name ENOFILE would have been too narrow(狹隘) in its meaning.