Baseball Question

[HajiShirazu]

Why is it that these days, people commonly talk about starting pitchers being "overworked" if they run pitch counts over 100 or pitch past the seventh inning, when just 30 or 40 years ago, it was common to pitch on three or even two days rest and going into the ninth inning was the norm? There is simply no way that people's arms have simply changed such that they can't throw as many pitches, in fact, today supposedly athletes are far better conditioned and should be able to go longer.
I know there was less offense, so were pitch counts lower in those days? Or was it simply because back then pitcher attrition was so high that people eventually figured out that one could only stay healthy throwing 90 pitches a game every fifth day. Yet pitchers today seem to still be getting injured as much as ever, so I guess it isn't working. Or is it that modern managers/statistics indicate that a bullpen guy is better than the average starter in the 7th-9th innings? With so much more scoring, this doesn't seem right either. Or are pitchers today putting more energy in each pitch and hence really can't go as long? I don't get it.