Supply and demand.
I know three different women who graduated with bachelors degrees in biology (different specialties). None ended up in biology as a career. One became an elementary school teacher (not bio), one became a nurse, the other went into IT. Why? They all discovered that their discipline is oversupplied with college grads, and so the expectation is that you will serve SEVERAL YEARS of unpaid internships to get enough work experience to separate yourself from the pack before you are employable….and get paid peanuts (depending on specific career path…some bio folks can get paid well…but many do not).
I work in IT. With the exception of those jobs where clearance is a problem, everywhere I’ve worked LOVES to hire interns, pay them handsomely and train them up. Why? Because IT people, even at the internet level, are RARE relative to demand. So training them up increases the chance that they’ll stick around our of a feeling of gratitude and comfort. Even if it only works 1 time out of 3, it’s worth it.
Now, each profession is going to be a bit different in detail, but I’ll bet if you really drill down, the main differentiator is supply and demand within that specific market/situation.