Interesting question...
If you find a passion for teaching math later in life, there are multiple routes you can take. You have to take a test and go through other loopholes to teach in public school, sure, but not so for private schools. You can even tutor all day long (and make good $$, too). You're considered a teacher no matter which way you go.... and you sure don't need a formal post-bacc program to be a fine educator. Practicing medicine without any examination or license... good luck! If you want to be recognized as a doctor, there's only one route you can take, and that route is obtaining a medical license. Let's explore this latter route further.
Assuming your passion for medicine comes after college, what can you do? The people who hand out those medical licenses make you take certain classes and have certain experiences; if you don't have these classes and experiences in some form, you don't get the opportunity to practice medicine. Some of these "experiences" (for want of a better term) lead to previously unknown volunteer opps, a leg up at certain schools, even automatic interviews and acceptances. Since the supply of students wanting to be doctors is greater than the number of available slots, these students are willing to do certain things to increase their odds of acceptance, often at great expense (both personal and financial).
Fortunately for students, post-bacc programs are lucrative for universities. Really, what is the marginal cost of adding a single student to a 100 person lecture? Not that much, considering that the student may be willing to pay upwards of 3K for the most expensive version of that semester lecture. Find 20 kids willing to pay 25K/year for tuition, hire an advisor for 50K/year, throw in some profs, and you can see how profitable these programs are. There are more factors to consider, of course, but there's no denying these programs make money.
That's not to bad-mouth post-bacc programs in any way; I think they're great. But you can see how the supply and demand converge to create a pleathora of post-bacc programs across the country, all due to the monopoly of the process of getting that medical license.