As a former player, hs then a year in the minors (low low A), pitchers and catchers work the most cardio on the field and the time in the dugout before going out again is not enough to recoup. As for the other 7 players, you’re in a game where you might have to go from stationary to a full blown maximum sprint in a nanosecond which may include a maximum burst of energy like throwing the ball. Do this between 5 or 10 times in a 3 hour game, 162 games a year, it will eventually take a toll. Remember that the player is on a field where they have to handle an object traveling between 50 to 110 mph with no notice. The worst part is the constant anticipation of having to instantly react and then not having to react. The mental strain, especially at that level, must be horrendous.