If you include campaigns that aren't completed, I can see that average being accurate-ish. Just this past year, my online group, with a single DM, went through four different "campaigns", but none were really complete. This was due to either it being centered around a single published adventure, and we wrapped up, or the new campaign wasn't shaking out like the DM or players hoped. I know I've played in many other abruptly ended campaigns over my time in the hobby.
As a GM, I often suffer from having the itch to play the new idea I have for a game while running my current campaign. Sometimes it's system fatigue, sometimes it's just impatience. I usually resist, though.
That said, my other group is alternating between two different campaigns on a bi-weekly basis, one D&D 5E and one Savage Worlds. So each game gets played and average of once a month. The D&D game is going on probably around 50 sessions, and the SW is closer to 30.