Because it is scrolling. If they would show one page of text while filling the next one in the background, the result would probably be somewhat like flicking through a book at one page per second. You still can not read one page per second but you would not be limited by your eyes being unable to recognizing the quickly scrolling text.

EDIT: As others have pointed out and I now did some reading on, it is an illusion that you can see all the text on a page at once, that is beyond the resolution limit of the human eye. To actually see all the words, you have to scan the page and that takes several seconds. From the numbers I have seen, it seems that the ultimate limit is probably below 30 tokens per second, no matter what, even using rapid serial visual presentation to cut out eye movements. Even 10 to 20 tokens per second is probably pushing it and unsustainable for many, if not most, people.