Why do people get an MBA quickly after graduating from college with a mickey mouse degree?

I've gone through the biographies of some succesful business-people like Leon Black and Bill Ackman and I'ven noticed that they go to college for degrees like history, philosophy or social studies and than immediatly get an MBA to enter business. If you just wanted to go into business, why wouldn't you just get a bachelor's in business?

I thought that the purpose of an mba is too break into the business side of your industry after you have worked in it for a while.