Why are login forms centered?

Why are login forms always centered on the screen? Most horizontally, some vertically also. Sites like Apple/Amazon/Adobe/Microsoft/Google/PayPal/Instagram/Twitter etc. are all centered! Is this an undocumented best practice everyone unanimously decided to use?

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Obviously it's most aesthetically pleasing, but is there any other reasons why?

Edit: Many of these sites do not support mobile either, so it wasn't done specifically for that.