The weird thing is that it doesn’t seem like they are pushing themselves harder. Pitchers I can see since it’s all about velocity and spin rate now, but position players get platooned and pinch hit/pinch run for a lot more now than they used to. There aren’t a whole bunch of players playing 155+ games each year any more. On top of that, you don’t have nearly the amount of guys bunting and stealing a lot of bases anymore either.
Granted, guys aren’t popping pain pills like they probably did in the 80s and 90s either….