Tom Tango notes that WAR is not proportional to IP, which is why great pitchers of today do about as well in WAR as the great pitcher of yesteryear, despite fewer innings:

You see, it doesn’t matter, necessarily how many innings you throw.  What matters is how well you can pitch compared to the quadruple-A pitcher.  And if you can improve your performance on a per pitch basis and throw fewer pitches, the two combined call allow you to maintain the same level of value.  

All that’s happened is that the superstars of today have shed their “replacement level” innings, which allows them to improve their rate stats (while dropping their innings). And so their quality X quantity is maintained.

TangoTiger.com

I don’t think the explanation is complete. Those wins have to come from somewhere, and the main rate stat that improved was strikeouts. More strikeouts means less work for the fielders, so I have to believe those gained wins are coming at the expense of fielders. It’s not just higher quality in fewer innings, it’s taking the ball out of the gloves of fielders. If the K rate had been held steady, would we be seeing WAR proportional to IP?

I wonder if defensive WAR is down in the same time frame? Fielders are much better than when I was young, so is the quality X quantity maintained there as well?