I'm no expert. Pitched maybe twice in High School, never played at a higher level so take the following for what it's worth.
"In one particular 8-year stretch". ...
"-from 1991-98 – the Braves split 6 Cy Young Awards, between Maddux (3), Glavine (2), and Smoltz (1).
The Braves also won 7 National League East titles, and advanced to 7 National League Championship Series and 4 World Series, winning one, in that same stretch."
I left out a sentence in between the quotes above. It is the common denominator between those three Hall of Fame pitchers.
I will agree rest is needed, after the season, for 3-4 months. Not in season or leading up as well.
Someone once said this...
"I have talent. I can't explain it. You either have it, or you don't."
I believe that to be the ultimate reason. Everyone else is just trying to keep up.
Jmo