I think it comes down to 2 different personality types. First, you have the people who would score 24 and think "oh man, I can do better! Better register for the next test and buy a bunch of practice material!"
And then you have the others who get a 24 and say, "Nope. @#$% this. I'm going to law school."
...And don't forget that some people mistakenly think the MCAT is "easy" (these are generally the ones who take no practice tests, get a 12, and then can't figure out what went wrong). And there are others who schedule it at a time when they have 5 classes and a full-time job. And others who decide to take the test before finishing the prerequisites...
Just remember that the average for those who apply is 28, and the average for those who are accepted is 31.