What I find bizarre are the number of people who go up to the flying club (post PPL) and simply tootle around the local area for 45 minutes every couple of weeks. Post PPL, flying can still be fun, interesting, and entertaining, even using club aircraft. I fly with another PPL which does help reduce the costs a bit, but seven years since completing a PPL we've both still no intention of giving up, we're both still flying as regularly as anyone else, and we're still finding new places to go and things to do. Most of those people we've seen over the last seven years or so, who just fly round the local area once in a while, drop out sooner or later.
I think a fair number of people learn to fly because it sounds like an interesting thing to do. Lets be honest though, whilst you're doing a PPL course, you're pretty much spoon fed - do this, go there, etc. Post PPL, all of a sudden you have to make a bit of an effort to find new and different things to do, and those people who don't want to make that effort, can't be bothered, drop out sooner or later, probably fairly soon after they've taken all of their friends and families flying.
Andy