I believe that most DMs have only run a single campaign world, whether it was one big campaign that has been continued through various ages, or they’ve only managed to run one satisfactory campaign. The tendency for a DM seems to be to conserve the number of campaigns they run by reusing worlds or tying them together so that in the end the number of campaign worlds they run is as close to one as possible. I think our tendency as DMs is to keep things relatively stable within our game worlds unless they are split by something like a change in campaign.