There are stories worth reading on both SOL and Fine Stories. For some values of "great author" there are great authors here. Top fifty lists have most of the top stories and therefor authors. Not every author on the top fifty list is a "Great Author" even in the sense Robert Heinlein was a Great Science Fiction author. I'm not sure even Nobel prizes indicate the recipient was a "Great Author". I suppose there are some names like Shakespeare or Voltaire or maybe Charles Dickens that qualify as Great Authors. Likely you can think of others that get recommended by English teachers.
What we have here are very good, well worth reading authors. At least some of them are. If I misunderstand what you mean by "great author" I apologize. There are slightly more than 35,000 stories posted, and since most authors write more than one story, probably less than 10,000 authors. The Bell curve or Normal curve indicates that about two thirds of all authors will be within one standard deviation of the mean (average for normal samples). Of the remaining third, half will be below one standard deviation and the other half (one sixth) above it. I don't know how many standard deviations above the mean it takes to become "great" but likely it is more than two, so perhaps a fraction of one percent might be great. One percent of 10,000 authors is 100 authors. There are probably less than 50 "great authors" posting here. Maybe far less, or even zero, depending on what your criteria is for a great writer. I would guess most writers in the running for "great author" have made significant income writing. I am afraid that hurdle lets out most of our favorite authors here. That doesn't mean there are not a lot of good stories, well worth reading here. It just means there aren't very many great authors in any generation.