Why is it assumed fighters from 12-15 round eras cant go 45rounds?

Whether it was 42 rounds or 40 and 85 punches per round each or cumulativley it was still a respectable workrate for a 12 round fight today over 40+ rounds.

 

I think for the old timers endurance and what they called "gameness" was the most important quality. Since people worked all the time, except on holidays, the big fights were often on July 4th or Labor Day, and the men fought sometimes all afternoon in the hot sun. Those fighters were really stout on endurance.

 

A lot of those long fights were snorefests too. They often had arm/hand injuries in these marathons, and after that were barely throwing any punches at all.