It's designed around the idea that the user chooses the width of their browser and the page becomes as long as necessary to fit all that content by scrolling, just like a word document. Centering, especially vertical centering, is kind of hard to define in that context.

When you start adding features to give web developers pixel-perfect control of the viewport then you impede other features like allowing the user to choose their own font sizes while still having everything render in a sensible way.