This is another problem I see with boxing and a further reason why MMA is burying it. Top boxers never seem to face other top boxers and when it happens, it's often a borefest due to the caution shown by both (Mayweather vs Pacman, Fury vs Klitchsko, Quigg vs Frampton). The lack of big fights may partly be down to promoters wanting to build hype but I do feel some fighters genuinely just want to keep their records perfect and are thus happy fighting nobodies.
Go into MMA and the top fighters are always wanting the other top fighters, everyone calls out McGregor because he's the money fight, the Diaz brothers will scrap anyone (inside and outside the cage

), Cowboy Cerrone signs any fight contract put in front of him, Jon Jones (when not banned) and Demetrious Johnson have cleaned out their divisions full of world class fighters and there's so many guys on the UFC (& Bellator/ WSOF) roster that are constantly wanting a fight asap... You put most of these boxers in MMA and their mindset would be to fight 10-15 nobodies before challenging anyone in the top 15.