Think about it this way.
If you have a solid plan with solid materials, why would you post? You did your research, reading through tons of strategies to find the one that fits you best. No need to post, right?
Then you take it and you bomb it. Everything you thought you knew is apparently wrong, and you have no idea where to go from there. Needing help, you post.
Then of course are all the lurkers who just read read read and never post, whether or not they have an issue. Taking purely posters as your sample introduces so many biases that it cannot be an accurate representation of the population of MCAT test takers as a whole.
Also, think of it this way: it's matriculating students. That means these are the people who did have the drive and push to properly prepare for the application process so that they get the big envelope in the end. Talking 3.2-4.0 GPA, great LORs, great ECs, great PS, great secondaries, great interview, AND great MCAT. They have absolutely done the prep and the research to do well and in the light of this, it isn't surprising that many of the students who matriculate only take it once. They have it figured out, and there's no reason for them to post unless they are trying to be helpful to others who may be struggling.
Those who take it twice or thrice or multiple times are those who had hiccups along the way. Made mistakes. It happens. And at 37 percent of the matriculant population, that's roughly 1 out of 3 people. Still seems reasonable to me.