37%. I would think it is much higher than that.
I think there are a few reasons:
1. Players believe they are going to continue, but then life happens. Someone has a kid, someone gets married, or someone moves. Then, the group just slowly dissolves from there. Maybe the one person was the social glue? Or maybe they were the GM? Or maybe they had the house everyone played at?
2. People commit without a plan. A plan might be: "We are playing every Sunday from 5Pm to 9PM." Or it might be: "We are meeting at Rita's house every time we play." But so many groups are just trying to schedule randomly or "choosing" where to play. Then, suddenly they are surprised when they said on Sunday they were going to meet on Friday, but Friday came and Terry forgot to tell his wife he was playing, and she expected him to watch the kids because it was girl's night. Things like that happen all the time without a consistent plan in place. (Heck, they happen even when you do have a plan in place, but at least it happens less often.)
3. People burn out. Players do, but the GM can really burn out. And if they are the only one's running... well... the game ends.
4. (It wouldn't be my post without me mention this...) The GM doesn't prep properly. The GM runs a great campaign for the first five sessions. They did their homework. They had interesting NPCs. They had the gaps filled. They catered to their players' wants. Then, they stopped prepping. They begin to think they can "wing it." And, in turn, it becomes less interesting for the players. Then, the game slowly fades because there isn't a clear ending point.
In my opinion, these are the top four reasons. I like the thought of people moving on to other games too. The whole, "Ooh look, something new and shiny!" But, just from my experience, it only happens with groups that play a diverse number of games. And most tables I know generally only play one or two.