It's an interesting point as to why so many qualified PPLs drop out of aviation - but I think the 3 main reasons are money, money & money. The petty bureaucracy of the CAA doesn't help of course, but as GA is such a small community, there's very little chance of any Gov't being remotely interested in proposals to expand our part of the aviation world.
The answer is for the CAA to be like the FAA and have a mandate to encourage aviation as well as regulate it. The only chance of that happening is if an aviation enthusiast somehow finds himself as a junior minister in the MOT, in charge of the CAA. (But as most people I've met in aviation tend not to be corrupt, self-important scumbags - there's very little chance of them becoming MPs).
Having said that, I can't believe people drop out through boredom. Even if you're not doing it for a Commercial license, there's so much you can do after your PPLs - type ratings, aerobatics, twins, etc. (Presuming you're as rich as Croesus, obviously).