I've never really understood this (not that I'm denying the pattern, it's definitely something that happens enough to be noticeable) - because do devs worry people will stop supporting them just because they complete their first project and begin the next one?
That can usually be a concern in general. Does a creator potentially risk alienating their fanbase, especially if they deviate from their initial -successful- title?
Sometimes it's the audience itself that requests/hopes/demands that the game goes on forever, Maybe because they're attached to the characters, maybe because they want to see their expectations or anticipations fulfilled, or any other reason.
And sometimes it comes simply easier to keep adding stuff instead of converging into a closure. Not because of laziness, but more because of reaching a point of "how the fuck do I end this properly".
And between abandoning and continuing it until they die of boredom (while making money), the latter might seem a more appealing choice.
TL;DR, eternal development is a damn easy rabbit hole to stumble into.