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.
In my experience of giving code reviews, I don't see how you would be able to achieve the same quality with just a high-level review. Coding standards and uniformity in code bases are so important, and you just can't achieve that with the high-level review
Eh, I personally find GitHub more useful for line-by-line nitpicking than for high-level, architectural reviews. GitHub PR diffs encourage reading code in an arbitrary top-to-bottom order and don't make it easy to jump around and get the bigger picture.
Unless you mean that they don't place any importance on the diff at all, which I might be inclined to believe, but that leaves a lot of the onus on accurate, well-written PR descriptions… which not every developer seems to be capable of.
