Because it often doesn't work right, and there's no way to tell. One of the most common complaints we get is that things "are slow to start" or that "I click a

Because it often doesn't work right, and there's no way to tell. One of the most common complaints we get is that things "are slow to start" or that "I click and it's slow to respond". After long and expensive remote diagnosis, this turns out to be slow DNS, and 8.8.8.8 fixes it. Falling back to it wouldn't change the user experience.