I agree that there is a lot of turnover at agencies and that girls leave agencies for a variety of reasons: some leave because they don't like the agency taking their cut and others leave because they don't like management's approach (which could mean a lot of different things). I will clarify my point(s)/thought(s):
1) If you drill down and look at most of the girls who have reviews under the umbrella of an agency, then generally speaking (at least for agencies in the DC area), the range of reviews for the girls is reasonably tight (maybe ~ 7.5 to ~8.9). Compare that to the average girl on backpage or even this site where the reviews are all over the place.
2) It has been my experience that agencies (at least the good ones) genuinely care that the customer was satisfied. Often times, immediately after the encounter an agency will text me to ensure that I had a quality experience. That seems to translate to the girls they employ really caring about the experience. Rarely have I had an independent do the same (text me afterwards to ensure that I had a quality experience).
3) Independents value their independence and flexibility of scheduling. Based on my experience, though, independents are far more likely to cancel or reschedule an appointment last minute, not be ready at the scheduled time, and/or be more aggressive. I'm all for empowerment, but I suspect that some women may behave differently when they know that an agency is following-up with clients and ensuring that they are pleased with the experience.
4) If an independent gets a bad review, it is very easy for them to simply create a new profile and obtain a new phone number. That doesn't happen with agencies. Many of them value their brand (some independents do, too, obviously) and it is too burdensome for an established agency to start from scratch.
5) Your experience is an anomaly in the whole scheme of things. Consider that maybe (best guess) 75-80% of girls have average TER scores below the range given in #1 above. In other words, probably 75-80% of the ladies reviewed couldn't get into an agency (assuming that they wanted to join one). As for the other 20-25%, maybe 1/3 once worked for an agency, but left for one or more valid reasons.
Obviously there are fantastic independents and agency girls. Both routes can lead to gems. And obviously I am making several generalizations. These generalizations just underscore the importance of doing your homework.