It's probably because the first 80% is really easy to learn so you get all the gratification of "being smart", without ever having to do anything difficult.
It's actually because I tend to be very action-oriented so at some point I get fed up with all of the planning and have to dive in and do something. Then, at a certain point down the road, I realize that the resource requirements (typically time or money) to get to where I want to be are too high so I drop it. I actually get more frustration than gratification out of it.