Many jobs are personality types, not educational types (which is yet another reason why the educational system is whacked, but that's another story for another day).
That being said, there's numerous reasons why. One, of course is connections. Another, of course, is the aforementioned personality. It doesn't take an MBA to sell a product, it takes a person who is likable, and can properly advertise to a customer. My twin sister majored in English and has done very well in her sales roles. A friend of mine, on the other hand, majored in Marketing and has struggled (though he's also latched onto crappy companies).