Why are there so many Jacks in fairy tales (Jack Be Nimble, Jack & Jill, Jack and the Beanstalk, Jack Sprat, etc.).
-- Not Jack, the net
Hey, Joe/Bud/Mac/Bubba/John Doe. See? Lotsa general, catch-all, applies-to-anybody guy names out there. "Jack" is a nickname for the ever-so-common name "John." It's especially popular in England, the source of the Nimble/Beanstalk/Sprat Jacks you cite.