I'm designing a website with a login screen, so I've been looking around at examples on other sites to get a general feel for the UX/UI trends in the area (I'm primarily a dev, not a designer). I've noticed that sites that have headers/footers seem to lose those when you go to the login screen, and the only thing you see is a simple login form, centered (in either 1 or both axes), with a simple company logo.
I'm wondering if this is simply a stylistic choice to keep it simple, or if I'm missing a more nuanced reason for ditching the headers.
Is it bad to have headers/footers on the login page?
www.microsoft.ca -> has a header at the top, but the login screen is completely different, and lacking a header/footer
www.google.ca -> the top links are missing when you click on Sign in
www.amazon.ca -> again, the top nav is missing entirely (I sort of get this one, since their header is primarily search based, but this also applies to woot.com, which just has categories in their header).
wwww.github.com -> another one that presents a clean sign in page, although if you try to create a new account, the headers return