copy/paste from an article
"It's a convergence of factors. First, hitters are generally better all around; you have fewer glove guys on a team so there are fewer free outs (having the DH contributes here as well). Second, we have a ton of data about how pitcher usage affects injury risk. Pitchers, particularly young pitchers, who have heavy workloads tend to get injured at a higher rate than those whose workload is managed.
There will always be exceptions but teams don't have the benefit of hindsight. It's impossible to know ahead of time if a young pitcher is going to become Mark Prior or Bob Gibson, Doc Gooden or Roger Clemens. We don't have the benefit of hindsight. We can't weed out the ones who can handle high pitch counts from the ones who can't with anything approaching 100% certainty. So the smart thing to do is manage workloads as well as possible to balance risk vs. reward.
Lastly, the pitchers you mentioned are hardly your average starters. They are the outliers, the names we remember because they were durable and outstanding for many years, thereby ensuring their place in public memory. If you dug deeper into baseball history, you'd find legions of young pitchers subjected to heavy workloads who fell apart during that period. We've just forgotten about them because they did not endure."
