Eh, read the whole thing. It's way more complicated than you're making it sound and it's not exactly like you're saying about the number of phrases, cause they use one phrase to predict the likelihood of the presence of another (one or more), which means amount of text/content on the page would have an influence on what phrases are expected and the expected rate of the phrase being repeated.
Saying a longer page that repeats a phrase or related phrase more than a static number of times it's usually repeated on a shorter page is not what they're doing or trying to do, for exactly the reason you state.
Another way of saying why they aren't and wouldn't do it that way is: It would be silly, overly simplistic and likely not patentable.
Example of Predictability or Expectancy Change by a Phrase:
The phrase "Comprehensive Information About Widgets" or something similar could (would in my opinion) impact the expectancy of the phrases you're stating would cause any longer page to tank, cause the expectancy or predictability of other related phrases would likely be impacted so the longer page wouldn't tank due to the predictability of those phrases in conjunction with a more "inclusive phrase about widgets" being present on the page and those phrases themselves would impact the expectancy or predictability of other related phrases on the page.