The problem is no one want's (or should) fight a guy who is way better or way worse than him
2 guys can both be undefeated but with very different record.
Like 21-0 vs 7-0 or 46-0
and those guys might be at very different levels in thier development as fighters.
Valuev is 46-0 but only really started being any good maybe in the last 20 fights.
A guy with a strong amatuer record might be quite good at 10-0.