I don't outline a short story on paper, but I could summarize it to you verbally (beginning, middle, end) before I am able to start writing it. I feel that if I don't do at least this much, I am making up a story rather than telling it, and I've yet to meet anybody--good storyteller or not, good improviser or not--who can make up a story on the fly as good as they can tell one they already know. .
I'd say you haven't met a lot of good short story writers. Pretty much every top short story writer I've met, or read about, makes up their stories on the fly.
The test of a good story is does it sell to a good magazine, and do the readers like it. I've met darned few short story writers in my life who can consistently sell short stories when they plan, think too long about, or take too long to write, their stories.
I can give you a yard long list of top short story writers who always write on the fly, who write their stories very fast, and who consistently sell them to top magazines.
I've written stories, start to finish, from first thought to final draft and submission, in four hours that sold for a thousand dollars and more. And years later, I can't find a word I'd change in those stories.
Nor do I edit the heck out of them. Neither do most of the other top writers I've known or read about. Like Robert Heinlein, many of my first draft are the ones that sell, though I usually do a quick second draft just to clean the story up, much as Isaac Asimov did.
Such things always remind me of how William Saroyan broke into print. He did so by write a short story a day for a month. One of those stories, written in a single day, was That Daring Young Man on the Flying Trapeze, a story that's about as good and as famous and as lasting as you can find.
Nor is it only short stories that top writers write in this fashion. I can also give you a yard long list of top novel writers whop write the same way, and who do so extremely well. Some of them often write a novel, a very successful novel, in less time than it takes many to think about and write a short story.
The last thing on earth I want to know is the beginning, the middle, and the end of any story I write, from short story to novel. For me, this means a story I'm almost certainly not going to sell, so I've learned it's also one I'm not going to write.
And, as Anne McCaffrey said, "If I already know how it ends, why would I want to write it?"
To paraphrase an old cliche, the proof is always in the publishing. If whatever method you use allows you to sell your stories to top magazines, keep doing it. If your stories aren't selling to top magazines, it's time to think about a change.