There are too many factors to list, but the bottom line is this: IF building was really as "enjoyable" as we all nostalgically like to think..then more guys would be doing it - and the RC model/kit business would still be thriving, providing us all with what we clamor for! The FACT is, many guys built because it was the only way to get an airplane in the air, and NOT because they were so in love with the process.
When the first ARFS were introduced (Lanier, SureFlight, Byron) they would have been soundly rejected if what I am saying was incorrect.
A secondary reason guys ARF instead of building is that it allows them to put many of their favorite aircraft in the air while also pursuing other things in life; other hobbies, family interests, job, home projects, etc.
Given the choice between months devoted to one single endeavor (a model) and being able to also do many of the other things we like to do, most of us will choose the latter.