Weight training was not as prevalent as it is now and the training is better. The food might be the same, but the diets and dietitians are better. Therapy/rehab and medical science have all improved and video review technology is so far more efficient and readily available. Guys used to use spring training to get into shape. Players may take October off, but are back training by Thanksgiving.
Another thing, the rosters and positional prototypes have evolved and there is far more specialization now. Dan Quisenberry broke the 10 year single season save mark in 1983. 22 others have a combined 72 seasons with 45 saves or more since then. The 3-true outcomes hitter was rare and strikeouts were to be avoided. Now batters are less protecting the plate and swinging for the fences, sitting on the fastball, with two strikes. A baserunner is worse for the pitcher. Pitchers akin to Greg Maddox and Jamie Moyer were far more the norm than the fireballers like Roger Clemens, who would only top out around 93. A LOOGYs split-finger is 93 nowadays.