There were also lots of pitchers who threw a ton of innings every single year and didn't go on the IL.
In 1969, 59 pitchers threw over 200 innings, 12 over 290 IP. By my count, 11 were Hall of Famers, and a lot of others had long careers.
Maybe they weren't throwing 90 mph, but you better believe they were all throwing as hard as they could. Why were men more durable then (I still let them play catch on my lawn)?