... at least for those we can prove. There are non-human writers, by the way. These include a few computers, and most likely a few higher primates (writing very simple works, granted, and probably dictating).
People prefer characters they can relate to. This is why most fictional characters are H. sapiens instead of, say, T. rex or A. oryzae.
Besides the large number of stories concerning humans [...] [tvtropes.org]