Why is the 4 week sprint still the literary default?

I’ve been wondering for a long time why the 4 week sprint still seems to be the default in Scrum literature. Even the State of Scrum Report states there is a 38% majority using 2 week sprint while 29% use 3-4 week sprints (page 25). Given that 3 and 4 week sprints have been merged in the statistics implies that the actual percentage amount of teams using 4 week sprints is even lower than 29%. Yet in the same report insight #2 states that “a Sprint is one iteration of a month or less that is of consistent length throughout a development effort.” completely ignoring its own results (page 38). Also, why isn’t the book “Software in 30 days”, released in 2012, called “Software in 14 days”?

Part of Scrum and agile in general is to generate feedback as quickly and often as possible, using 30 day sprints you spend a whole lot of time between two feedback cycles. In addition 4 weeks of time is so much that it’s really hard to look back at them when sitting in a retrospective. Can Scrum literature please inspect & adapt and use the 2 week sprint as the new default?