Because I think some boxers need that opponent they know they can loose to, to be motivated and be at their best. It's not that they consciously take these other opponents lightly necessarily they may still go thru the same training camp and what not, but subconciously there is probably something in the back of their head that says that they can beat this lesser opponent easily, where as if they fight a top guy they KNOW they better be at their best or they won't win.