Serious question, though consciously naïve: how could boxers from other eras fight SO MUCH?

I have seen this in amateur boxing, and recently so. One kid in my gym was sent from one tournament to another, so he was basically fighting 3 weekends in a row. During the weekday training he did not do any sort of sparring at all, just worked on drills and mitts.

Its actually not difficult to fight every weekend, I see it in our Friday sparring sessions. There are guys who barely train at all, but show up every Friday to do some sparring, which often ends with some action packed rounds to finish it off. Usually we do 8 or 10 rounds, but these guys go for 12.
So as long as you are not injured, its not that difficult. I will never understand why guys dont sprinkle in more easy fights into their schedules. Keeps them sharp and earns a quick buck.