So no-one has an answer to my original question?
I mulled this question over the last week while doing other research. I have asked this question as well. I poked around to see if this dissertation had been cited in the Forum, and apparently not:
Murray, D. W. (2011). Home brewing and serious leisure: A mixed methods examination (Doctoral dissertation, Auburn University).
Murray states that past the beginner level, motivation to continue a hobby includes "the requirement that the activity involves overcoming hardships and difficulty, often necessitated in attaining competence coupled with a significant investment in goal-related behavior over time" (p. 18). I've seen similar claims in other scholarly materials related to motivation for leisure activities.
If you go back to the first time you bottled, you were probably excited that you were actually making beer! In bottles! But it likely didn't take long for your interest in bottling to plateau. Once you get past basic issues such as sanitation, measuring sugar by weight versus volume, using a capper, etc., bottling is... bottling. The laborious "laundry" of homebrewing.
There's plenty to suggest on the Forum and elsewhere that kegging requires a level of expertise that separates keggers from homebrew beginners, which itself is motivation enough for a hobbyist. This week, I'm excited that I appear to have no leaks or other errors in my first kegging attempt -- to the point where when I get home from work or even a shopping trip I make a beeline to look at the regulator, and have repeatedly tested the beer despite my intention to "set and forget" for at least a week. I am not sure what comes after kegging, except more kegs, but since it lacks the time-intensive drudgework of bottling, there's no *de*-motivating factor. But meanwhile, the frustration appears to be actually part of the motivation.
Sorry for this nerdy response... literature searches on this have been a side hobby for me this week.