I don't know the real answer, but my guess is: Pitchers these days pitch harder, train harder, and put more spin on the ball (putting more load on their arm/shoulder). Also, there are more tactical usage of pitchers (putting in someone to get the next out and that's it).

The tactical swapping is part of it, especially since once a pitcher is taken out they can’t go back in for the rest of the game. A lot of the roster pitchers are backups and relievers who don’t play many innings. The starters usually take at least 3 days rest after pitching.