Couple things going on:
1) Pitchers used to “coast” - now that pretty much have to throw at maximum effort all the time just to survive.
2) We’ve forgotten the names of all the pitchers that lasted a couple of seasons and then blew their arms out throwing really high pitch counts, and remember the freaks (Spahn, Marichal) who didn’t
Well, that’s a theory (not one I necessarily agree with) anyway.
My Dad actually played on that Giants team (a year earlier before he was traded) and came up with all those guys (Marichal, McCovey, Cepeda, Gaylord, 3 Alous, Tom Haller, on and on). The game was played differently.
My Dad’s last season salary was $5600 I think.
I just checked baseball reference for the 1963 season. It seems virtually every NL team had one or more pitchers who had at least 10 complete games. Even the awful teams like the Mets. I think the Giants had all four starters with at least 10 complete games.