I've been delving into the history of baseball recently and one thing that has not been stated thus far (in any depth I suppose) was the dead-ball era.
Before Ruth's era, teams did not score runs in droves. Winning a ball game 1-0 was much more common than a blowout.
Borrowed this stat from wiki,
The nadir of the dead ball-era was around 1907 and 1908, with a league-wide batting average of .239, slugging average of .306 and ERA under 2.40. That year, the Chicago White Sox hit three home runs for the entire season, yet they finished 88–64, just a couple of games from winning the pennant.
There were some complaints about the low-scoring games and baseball looked to remedy the situation. In 1909, Ben Shibe invented the cork-centered ball, a ball which the Reach Company, the official ball supplier to the American League (AL), began marketing. Spalding, the ball supplier to the National League (NL), followed suit with its own cork-center ball. The change in the ball had a dramatic impact on both leagues. In 1910, the AL batting average was .243; in 1911, it rose to .273. The NL saw a jump in the league batting average from .256 in 1910 to .272 in 1912.
Small ball was the name of the game. Stolen bases back then were the homeruns of today. Guys didn't hit 20, 30, 40, 50 or 60 hr's, ever, back then.
And speaking of the ball, previously mentioned in this thread but I think we should look at it a bit further. Could you imagine a ball that was soft after a few innings. You could have the best batter ever, but with a ball that has been beaten on, without (until 1909) a cork center, there is not much you could do with it.
And my final thought is the sheer size of the stadiums back then.
Huntington Avenue Grounds, home of the Boston Americans has the most ridiculous dimensions I have ever seen. You think Adrian Gonzalez has it bad in San Diego? How about 635 feet to center. I mean are you kidding me? The game was a lot different back then, and the big bats were really non-existent until the early 20's.
So with that, it is total plausible to see a guy pitch every other day and reach the milestone that Cy did. It is not meant to play down his accomplishment, he obviously was great in his own right, but putting it in that scope makes it much easier to believe. Probably throwing mid 80's gas, I have nothing to back that up with, and putting very little stress (versus what we have today) on a throwing arm a pitcher could much more easily accomplish this.
Also before I forget, the idea of a bullpen didn't come until much later, so you either pitched the whole game or you were not a pitcher.