So, thinking as I write, why do most f/w PPLs give up? Maybe it's because, after learning, f/w flying just isn't that exciting.
I believe you make an error in confusing "f/w flying" with "flying airplanes". I respectfully point out that "f/w flying" includes sailplanes, and that cross-country soaring is much more challenging than the "set cruise power and trim it out" experience of cross-country flying in airplanes.
You take off, trim it out, look at the view, then have all the skill and excitement at the end when you come to land. OK, you can go in for aerobatics. You can learn to fly tailwheel or other more interesting aircraft. You can get an IMC, you can get into long distance touring. But if you've got a plain vanilla PPL, perhaps the reason you give up is that it's actually quite boring.
Yes, I agree. It is rather a shame that most people don't go beyond the basic PPL, and restrict their flying to boring holes in the sky in spam cans (evidence of the lack of personal initiative in contemporary society?). The
opportunities are there, if one only looks for them. In addition to the examples you listed, other that come to mind include float flying;
precision flying; instructing; multi-engine flying; formation flying;
air racing;
ski-flying; and
jet flying. No one person can ever become proficient at all aspects of f/w flying (show me a f/w pilot who's bored, and I'll show you someone who is not flying at their full potential).