Question about the early 70's...?

Did they increase the number of starts per year for pitchers or something becasue I noticed that many players from that time pitched 300+ innings which is odd because that feat is usually rare and was common among work horses then. Steve Carlton, Vida Blue, Catfish Hunter, Nolan Ryan, Mickey Lolich to name a little. And Lolich had 375 IP and 45 starts, how is this possible out of the deadball era?