Problem with the short stories is that it is difficult for them to be noticed.
This applies both for this site, and the Royal Road too.
This is because the vast majority of readers find your story through the Latest Updates section, and short stories, having only a handful of chapters (or even a single chapter) are restricted to the very limited time window to be visible there. More chapters you have, higher the chance that the random user could spot your story in that list. Additionally, monetized web novels are essentially a subscription service, there is the incentive to make them endless.