Medical school is difficult to get into. Think about all the people that start in college as "Pre Med". The premed club table is the busiest one during freshman orientation. Engineers, science majors, physiology, musicians, etc. they all want to be on the premed pathway. Then reality sets in. Of all of those hundreds of thousands of dreamers only 85k or so take the MCAT every year, some of which (many?) are repeat takers. The mean (old) score is 25, which won't give much of a chance anywhere. Of that group of 85k only about half apply, and of that group less than half are successful in getting an acceptance.
So one could say that many applicants are weak to start and nearly 1/2 the applicants are successful, so it's not that hard, especially if you're a pretty good candidate. And that's true. Apply with a 3.6+ and 31+ and you're in pretty good shape, but you're ~1 SD greater than the mean. Your chances should be pretty good.
However the reality is that there is massive self selection at every step of the way. People don't have the grades, can't handle the science classes, bomb the MCAT, lose motivation, etc.
Then others are lured away by easier careers.
Very few that take the first step finish the marathon.
That self selection continues at the residency and fellowship level as well.
However, the journey of 1000 miles begins with a single step, so get to steppin'.