When I started watching baseball pitchers would go at least 6 innings then give the ball to a reliever. Heck before I was even thought of pitchers wouldn't get off the mound until their shoulder gave up and yet there were less injuries. Now a days pitchers are getting paid way more than they're worth and can't (or won't be allowed to) make it past the 5th inning. The evolution of the bullpen has destroyed the modern day starting pitcher. Going into the 5th inning the bullpen is most likely already stirring with guys stretching and playing long toss if not already getting warm. While the starting pitcher is still in the game and not even in trouble. Unless you're the likes of Clayton Kershaw, Madison Bumgardner, or Max Scherzer getting through the 5th inning is a rarity if there isn't a no hitter or perfect game going.
Stat nerds would tell you that after the 5th inning pitchers are most likely to get roughed up because that's most likely the pitcher's 3rd time facing the opposing team's lineup, but isn't the point of Spring Training and preseason pitching camps (that most teams hold before Spring Training) to get pitchers stretched out and ready to pitch deep into games? More and more as bullpens continue to evolve and starters are continued to be babied eventually there's going to come a day when a team is going to have just a huge bullpen and no starters. The death of starters, in my opinion, has begun.