Why do so many campaigns never finish? Genuinely curious what others think

[Toe Knee Nose]

And since this is ttrpg general, not d&d, some games don't have levels.

I ran WoD game for 1.5 year. It was inspired by Supernatural early seasons "monster of the week" format and Dresden Files books. It was 6 players + ST (me) group. They started as normal humans that slowly dipped more and more into WoD and then, inspired by Block by bloody block supplement, game changed more into city territory reclaiming and protecting normal people from predators. By the end, we had 3 Hunters, 2 Mages and one Werewolf. Most sessions were standalone adventures, so it was very flexible scheduling wise.

Most sessions, they would get 2-3xp, and +4xp for every "milestone" they achieved. So after 60 4-5h long sessions, they had around 160xp on average ( some more, some less, depending how many sessions someone missed). It may seem a lot, but when for example raising STR from 3 to 4 costs 20xp and from 3 to 5 costs 45 xp cause you need to pay for every dot separately ( its new dot * 2 for merits, *3 for skill, *5 for attribute, *7 for supernatural ability), it does 2 things. First, it stops characters from becoming ridiculously overpowered. Second, it encourages horizontal growth more than vertical ( new skills, new merits). While characters don't necessarily get more powerful, players still feel and see their characters growing and evolving.

I can see longer campaigns when you don't have levels. Like you said, lots of games have a much, much flatter progression - progression tends to be broad rather than high. I can see that pretty easily. Games like GURPS or other percentile based games can also have really, really flat progressions as well. Although, thinking about it, I have never heard of anyone playing the same GURPS campaign for a decade. I'm sure it exists, but, it doesn't seem to be so much of a thing.