Why do many companies hide their navigation during the Sign-In and/or Sign-Up flow?
I have been tasked with designing a sign in flow, but I have noticed that many companies hide their site navigation during their sign in/sign on flows. I've seen quite a few articles on designing these flows, but none of them mention site navigation (take a look at Login Form..., 18 UX Design Tips..., UX Login..., 10 tips..., and Designing a...) You don't have to click on any of those because they all have the same basic information, none of which includes "hide your site navigation"
The examples that I have looked at include:
I have a lot of guesses, so let's steer away from speculation if possible, I'm looking for behavioral reasoning or data/numbers on conversion rates.
Thanks!
-Drew