Each medical school has a very low acceptance rate because the number of seats is very small. The majority of medical schools have 100-200 seats for each incoming class, unlike colleges which have thousands. Some schools have even fewer than 100 seats (although they're likely to get slightly fewer applications) while I think a few might creep up to about 250. So, if you take a school that has, say, 150 seats, and they accept ~2x that number (just hypothetically) for a total of 300, then you can see that receiving even just a little more than 3,000 applications will drop their acceptance rate down into the single digits.
Also, there are a handful of schools hovering near the 3.7/33 range for median accepted stats that receive up to 12,000 or even near 15,000 applications (Drexel, Jefferson, Tufts, BU, NYMC, Albany, Georgetown, etc), because a) many applicants who have similar stats are "competitive" for those schools, b) higher-stat applicants will apply to these schools as safeties, and c) these schools are within reach for lower-stat applicants. But again, remember that these schools usually have around 150-200 seats, so the acceptance rate will be very small.
If you take any one medical school applicant at random, his/her chance of being accepted at any one particular school is really small because each school has a very low acceptance rate (with the exception of in-state schools in particular states). But, if you know what you are doing as a pre-med and craft a solid application with competitive stats and meaningful experiences, your chances of getting an acceptance SOMEWHERE are pretty good as long as your school list is reasonable and you're not incompetent in some way during your interviews.