There are actually quite a few other benefits to this approach, I'll just rattle off a few.
An often overlooked part of UI design, because we think of digital as not being (as) limited by the human body. But what's more comfortable, looking straight forward, or looking to either the left or the right?
On phones, the middle of the screen is also considerably easier to reach than a side.
You want people to login, to look at your site. Centering the login creates as much buffer space as possible between the element you want used, and the world outside of the screen.
Consistency with other sites
Other sites do it, so you'll fit in nicely. While following the beaten path is not always the best option, in UX it can be good because it lowers the learning curve. This design is very archetypal, and people will instantly recognize the goal of the page and what to do next.
Consistency within variations
You can use the exact same layout for LTR and RTL text. You can load it on a phone or a PC and it will feel like the same layout. Equal whitespace fades into the periphery, but whitespace (or absence of it) on only one side makes a small or large site feel very different: