AFAICT this stems from a different philosophy of what a code review should be for. GitHub appears to believe that code reviews should be very high-level, with most discussion focused on overall design issues and only the latest version of the code, rather than detailed line-by-line reviews of incremental changes. This attitude is shared by at least some number of GitHub users, since many people who visit https://reviewable.io (my attempt at a code review tool in the vein of Phab or Gerrit) don't even get why it might be needed. So perhaps GitHub is not entirely wrong... but it certainly opens up a nice niche for providing tools to those who like more thorough reviews.