Recently a side comment came up about why modern pitchers don't start 40 games a season like they used to.  The question intrigued me so I did a little searching.  The results surprised me.  Mostly I used the Stathead tool at baseball-reference.com, and the most useful table I constructed is this:

https://stathead.com/tiny/SBDaM

Since the modern era, 1901 and forward, it's never been the case that pitchers in general were regularly making 40 starts.

For periods of years, the major league leader would regularly reach 40.  A few years, there would be more than just one, but never ever as many as there were teams, meaning less than one per team.  So it wasn't part of the job description, it was an achievement.

There was kind of a peak of 40-game starts around 1904-08, another 1914-17, then it picked up again after expansion in 1962 (when the season got a little longer), then quieted down and peaked again around 1973, then basically died out around 1979.  The last 40-game starter was knuckleballer Charlie Hough in 1987, who come to think of it had that in common with other "recent" 40-game-starters Phil Niekro and Wilbur Wood. 

In those 87 years, there were a total of 140 such pitcher-seasons.  One or two a year.  Zero since then of course.

The individual pitchers weren't doing it for years and years without end, either.  Only 31 such seasons were logged by pitchers over the age of 30 (despite the myth that that was when a player would enter his prime).  8 by anyone 35 or older.  Starting 40 was always a young man's game.

For another perspective, Sandy Koufax in 1965 holds the record for season strikeout percentage, 29.5%, among pitchers who started 40 games in a season (and of course Sandy led his entire league in that regard that year, among players who qualified for the ERA title).  By contrast, in 2019 there were 16 ERA qualifiers who had a higher percentage than that.  And even though Sandy was a "unicorn" of his era, and also a prototype for today's pitcher, he was finished before his 31st birthday.  Most of the guys who ever started 40 games weren't striking out the side.

Today it's 5-man rotations. Divided into 162 games, that's about 32 starts per season.

Used to be 4-man rotations. Divided into 162, that gives you 40. Divided into the older 154-game schedule, that's 38 or so.

Of course in really olden days, back into the 19th century, you might have 3 or even 2 workhorses who handled the bulk of the chores.  But back then the schedules could be more erratic too, and the game was just played differently.

Anyway, major league teams settled into an every-fourth-game routine a lot quicker than people sometimes remember.  Well, remember reading about.  That made 40 an uncommon feat.

Managers would love to have their best pitcher get as many starts as possible, so they'd be sending someone out 40 times if they could.  The tImes changed, not the intestinal fortitude of the players.