I don't pretend to have any knowledge of the inner-workings of modern day baseball. But, if I had to guess, it comes down to one word, money. Gibson probably never made $200K in a single season, while some of these pitchers make that per outing. I'd assume it's a combination of the player protecting his future, and management protecting their investment. But, I could be all wet.
My point is, if they keep themselves in shape so much, how come so many of then end up on the DL?