Well WB never injured his back and he fought tough people. Gold Roger never lost.
Ya and wb is fine. Since he was the strongest man in the world we all know he fought tough fights on his way to the top. Same with roger although roger tied with wb.
If someone like don kreig were to be undefeated it is likely he just sat back and didnt fight anyone serious.
For a real world example. Look at floyd mayweather, dude is undefeated and was at one point at the top of the boxing world. Now if i were to tell you i am undefeated..... in my one amateur fight.... that wouldnt be impressive right?