The simple fact is that the talent in ALL areas of the game is way higher than it was in the 70s and 80s. Batters hit the ball further, higher and run faster than ever before. Also ballparks are the smallest they’ve ever been (which a few exceptions). Back in the day it wasn’t uncommon to have a 500 foot outfield fence.
With more talented batters and overall stronger players, the smallest pitching mistakes end up giving up runs. Whereas before they were often long flyballs. Now it seems that because of the PED crackdown, batters are starting to come back down in power and the pitching really isn’t going anywhere because it is more important to have skill than sheer throwing strength when pitching.
Now none of that answers the question entirely… back in the day Jim Palmer (or whoever) used to play every fourth day and half the time complete the game… why doesn’t this happen now? It’s a combination of better batters, more effective relievers, pitch counts, surgery to extend careers, etc. And to answer the original question… it’s July, it’s the dog days… heat and the exertion of a long season catch up with a lot of starters about now.