There are several different approaches someone can take to choosing a direction for their webcomic. In many cases, the creators are doing something that readers can’t find elsewhere. Often, they’re looking around themselves and not seeing the type of comic they personally want to read, and come to the conclusion that they’ll have to make it themselves if they ever want to see it. That’s one of the great benefits to webcomics over traditional print publishing: if the people behind a comic simply have a story they’re compelled to create, posting it online gives them the opportunity to share it without the costs of print and distribution. The satisfaction is primarily (or perhaps entirely) in the creation process alone, and any idea of recouping costs is secondary.
Another direction, though, is that a creator may want to become a comic book professional and is hoping to hone their skills and/or garner the attention of a publisher, and use a webcomic as a means to that end. You tend not to hear much about these types of comics, though, and with good reason. Not that they’re all poorly done, necessarily, but they tend not to gain hefty followings in the first place and generally don’t stick around for very long in the second. And there are reasons for both of those things.
The reason they don’t become hugely popular is because there are frequently better versions of those comics already out there. Again, not that theirs are bad, but if the goal is to work on Batman, and the comic is something like “a cross between X-Files and Flash” then readers will see how it’s derivative and ignore it in favor of the original. Even though they’ve churned through hundreds, if not thousands, of creators over the years, the major publishers have largely figured out how to make the kinds of comics they already sell. A lone creator trying to emulate is only going to appear as an emulation, and one that still has to sort through its own mistakes. Why would a reader want an redundant comic that’s not even as good as the originals?
This actually then leads to the second issue. If the creator’s main impetus for making the comic is to gain attention, and their comic is not doing that (because it’s derivative) then it’s not fulfilling the creator’s goals. If it’s not really a story that a creator has to tell, but just one that s/he thinks will get them a gig, then there’s little motivation for them to keep working on it if that attention doesn’t come. Like anyone else, they’ll move on to something that ultimately does more for them.
It’s definitely not necessary to start a webcomic by trying to tell the story that you’re not seeing anywhere else. And while some creators have managed to launch notable careers in print thanks to their webcomic work, that’s hardly a guarantee and it only came about because the creator was passionate about the work itself first. Their motivation was intrinsic to what they were working on day-to-day (or week-to-week, or whatever their update schedule was) and wasn’t dependent on some future goal they were trying to obtain.