Magnetic credit card readers are cheap; look at how much Square charges. The fraud prevention mechanism for magnetic cards is mostly reactive and doesn't requi

Magnetic credit card readers are cheap; look at how much Square charges. The fraud prevention mechanism for magnetic cards is mostly reactive and doesn't require "passphrases" or "cryptographically signing," which is a big usability win. Since issuers are on the hook for unauthorized charges, and generally make money from transaction fees, it's in their best interest to find a good compromise between fraud protection and usability. Adding consumer software and a wide variety of commoditized portable computing devices into the mix doesn't seem worth the cost.