In the "old days" players didn't lift much. Players also weren't as big, fast, and strong - all of that leads to more stress on tendons and ligaments, which are critical pieces of a baseball player's anatomy. In addition, very few players made life-changing $, and pre-free agency the last thing an average everyday player wanted to do was sit out a few days or go on the DL with marginal injury that he could play through. Heck, Koufax probably pitched large portions of his career with arm injuries that would certainly require surgery in this day and age. Guys that got injured early in their careers typically didn't have long MLB careers, because they were replaced with someone off the farm back in the old days.