My ratio of DMing and completing campaigns was once inverse to my income. That is, in high school and college, I had a lot more free time and I was able to game longer sessions, with regularity, with my friends. Ergo, most everything was expected to be level 1-20. Epic metaplots. Even so, I only managed to get two AD&D groups to level 20 during this time.
Two.
After I took my first real job, I still tried to keep this expectation. Didn't happen. Not even close. People come and go. Jobs change. Kids, obligations, and so on.
Age of Worms? Close but no finish.
Savage Tides? Good try.
Kingmaker? Got to last module and key player went back to get a degree. This is, and will be, a period of constant change for most gamers.
I did try anti-burn out ideas such as a board-game or gamer movie day from time to time. That helps, but it wasn't going to help with people simply having to move on in life.
Now I'm settled, with kids and a family and a good job in a good city. I've sought out gamers who were similarly situated (age, career wise, got the kid/family thing figured out, not planning to move!, etc.). That means I'm not looking for casual, intermittent play with strangers at the game store. In doing so, I've increased my chances of finishing campaigns. Even so, it helps to not have every campaign go 1-20.
Pool of Radiance, level 7 max to try out 5E? Done.
Curse of Strahd, level 10 max, 1 year? Done.
Out of the Abyss, 1 year, doable and done.
Summary: After building a graveyard of half-complete campaign ideas, I've settled for realistic ones limited in scope while simultaneously seeking similarly situated persons. It's upped the odds of a satisfactory ending rather than "hey, we missed 4 straight sessions, let's roll up new characters and try something else..." It doesn't mean I won't try a level 20 adventure someday. But it has to be one helluva campaign.