True, there are plenty of "eh" volume-length narratives out there too. The difference: it is much easier to convey something (anything) with a longer piece than with a shorter one. This is why writing a strong five-paragraph essay is harder than a whatever 15-page paper. A short essay has to convey its point clearly, succinctly, and concisely. A 15-page paper can ramble away with the surety that some worthwhile nugget is buried somewhere inside it.
A good short story can't ramble--all the parts have to work seamlessly together. Likewise, a good manga short story comes down to a seamless interaction between text and imagery. A good manga short story with no arc, only a premise, relies almost entirely on its art.
To be continued . . .