A point I'd not considered. Still, if a pitcher is going into the sixth or seventh inning, still pitching strong and getting batters out, why change? I'm far from a baseball "expert" it just seems to me if a pitcher is still going strong why not leave him in? A good coach knows when a pitcher is getting tired and will replace them when needed.
Watching the last World Series for example was like watching one big pitching rotation in a single game. Dugouts got used up, pitchers were tired, etc.. How about some strategy instead of straight pitch-count?
This is most likely a combination of pitchers getting fatigued and batters getting more comfortable at the plate with more at-bats against the same pitcher.