The reason the old time guys fought a lot is because they were getting paid and paid well. Unlike today when even world ranked fighters have day jobs, there were a lot of fighters making good money. That was because there was no tv, no football to speak of, no NBA, etc...so people went to the fights and thus there were a lot of fight cards. That means a lot of work available for fighters.
I can guarantee you that you will never find a young pro starting out that would tell you he'd like to fight less. To a man (and I guess woman, these days) they are all discouraged at not being able to find fights and at getting paid peanuts when they do find work. The number of fights cards is not nearly what it was in the 1950s (with the advent of televised fights), much less the 1930s, and while there are a handful of fighters making huge money, most make nothing.
And a lot of the guys that do fight a lot and do get paid, use their fights to fight victims and pad their records, so they get to big fights 30-0 with 27kos, having fought 40 rounds. They may be "fresh" but they still fight like amateurs, which makes sense for a guy with only 4 fights worth of rounds under his belt.