I'm not sure if this applies to me necessarily - I am certainly HEAVILY research-focused. I wouldn't mind clinical work being a small part of my career within select specialty areas, but would never want to do it full-time. However if it ever becomes the "primary" activity of my career I will likely find something else to go into as I just cannot imagine that being remotely enjoyable for me.
I actually don't mind therapy as much as I did earlier in my program. I've gotten comfortable with it. I like to think I'm pretty good at it, a belief that is further reinforced every time I see what passes for therapy out in the "real world". Honestly, its not that its "terrible" its just that it is a much less exciting process to me than other things. Yes, every client is unique, but you don't have to do this for very long before you realize that they are somehow simultaneously all unique and all pretty much the same.
I like research because I like creativity. I like coming up with ideas and novel ways to test them. I like solving difficult problems that no one else has even tried to solve (or even recognizes as problems). I like the bigger-picture perspective research offers - I can help one person at a time, or I can do research that might help millions (not that its likely...but I can dream🙂 ). The aspects of clinical work I DO enjoy are the creative/higher-level aspects of it. Diagnostics, case conceptualization, and treatment planning are fun. Actually sitting in the room with the client explaining x, y or z, going over homework, etc.....meh. Same goes for assessment. Actually administering tests is fun the first time, but miserable after that. I hate it, I'm bored out of my friggin skull within 5 minutes, and for most tests I feel like my education is being wasted doing something any sufficiently motivated 14 year old could probably do just as well with minimal training. Interpreting the tests, integrating the results, etc. - that I actually enjoy. I obviously don't mind doing some clinical work and hope to always do at least a little of that to keep me grounded and clinically "fresh".
I don't think this is the end-all, be-all of it, but I also think the reinforcement process is different. Research certainly entails massive delays in gratification. One of my mentors routinely talks about a paper he now has in press that has taken over a DECADE to produce. From the time he thought of the idea, wrote the grant (it was a monstrously large one), did the study (recruitment problems, it took ~7.5 years), figured out how to analyze it, and then actually summed up the motivation to write it. That's a long time to wait. However at the end, the product is his. With clinical work, rewards (when they happen) tend to be more immediate. However, attributions are different. I maintain that good therapy outcomes are "usually" 90% the work of the client. While not intending to diminish our role, I generally find it harder to attribute the successes to what "I" did with clients. While I don't directly control the results of a study, I do directly control most other aspects of it and certainly the end product of it - which helps me attribute my effort to the success (even if that success is incredibly delayed). Different people likely weigh the costs and benefits of those things differently, but that's where I fall - it likely plays some role in those decisions.
I think I will cut it off there as I'm rambling at this point, but that's the gist of my feelings towards it. In sum: Don't mind clinical work, but like other stuff more. Many possible reasons why.