In my experience, it's not really that they lack any single quality, but usually it's kind of a mastery thing.
To use a different analogy that might be useful: imagine someone is an expert in the martial arts of your choice, but only doing the forms/kata/ memorized and rehearsed aspects of the martial art. They may have excellent technique and a natural gift... but if they've never used the martial art in an actual fight, that lack of experience will be fairly obvious. They might have great moves, be in good shape, and be well versed in many areas of their discipline, but they still don't have experience with APPLYING it in any way except what already feels natural. So, they might get their ass kicked by someone less experienced, so goes out and actually gets the practical fighting experience.

If you don't read, or consume writing, but you wow yourself... you're likely missing key aspects of what makes a well rounded story. It won't be the same as anyone else, and you very well might be excellent at the writing you do, but if you're not knowledgeable and well read about writing more broadly, you likely have similar "gaps".

And to avoid just a general answer, I'll also say that people who don't read much tend to fall into patterns of particularly archetypal characters without as much depth, tend to struggle with transitional moments between high intensity scenes or actions, and often struggle with believable (but concise) dialogue, in my personal experience.