Gus brought up the observation that one shots are unsatisfying because of a lack of character development, whereas long term campaigns are far more interesting.
This is very true in a sense; certainly, the longer the game, the more room there is for development and evolution.
However, in practice I often see the opposite trend. There is a sort of flip side to this observation:
In one shots (with games designed for that!) I see dramatic character development take place regularly. (Again, consider something like Witch or Dog Eat Dog, which reliably produces personal epiphanies and character transformations in every game.) In many “indefinite duration” campaigns, on the other hand, I see people shy away from development and evolution and stall progress in every way possible.
The result is that (in my experience) good short form games see a lot more character development per unit of time than the typical open ended campaign.
The best games are campaigns where everyone treats each session as a potential ending point or one shot, but the game continues due to sustained interest and enthusiasm. That’s like the best of both worlds, when it can be pulled off.