Series basically comes down to one main goal... money! It is also easier to write because they don't have to spend time creating a new world and new (well completely new) character sets and can just pick up where they left off. Even if the stories are unrelated, a large part of the work is world setting and character creation. Everything these days is about mass production and how fast you can release content. A series pretty much nails both.
For fantasy, that's just the way things go though. People want to read an epic adventure and not part ways with their favorite characters. That being said though, you can argue that The Hobbit does exactly as you are looking for. It tells of an epic story in a singular stand alone book that is unrelated to a series. Lord of the Rings series you can argue is a continuation from The Hobbit, however you can read both separate from each other without issue.
I would imagine that publishers generally want to know what your future plans are with stories. Everything these days wants to be turned into a franchise. Movies, Books, Shows, Clothing, Toys, Video Games, you name it they will try to ride the tails of your successful book. The more content you release for that story, the more they can in turn sell and market. That is also though how you would make your money as you only get something like a dollar per book sold after all is said and done. The more content and royalties you can hand out, the more you make off of your story too.
In the end, whether you have a series or a singular story, what matters is the content. You aren't going to sell a series if book one flops no more so than you won't sell a singular book if it flops. There is nothing wrong with writing non serialized stories! Just be prepared that you may get some fan mail demanding for you to continue the book with a new story so that they can see what other shenanigans your MC gets into. Quality over quantity will always prevail. There is only so many books to a series, movies to a series, shows to a series, that someone can write before it becomes stale and riding the wave of success as it's only driving force.