Because the user is typically looking at the center of a screen.
Sure, you can organize you windows in a way to compensate and center the content manually. But I think there are two usage patterns:
1. maximized browser windows
2. non maximized windows
For the first pattern, the above statement is essential. Naturally, the user is looking at the center of the screen. The second pattern is somewhat unpredictable, because you neither know where the window is on the screen nor the size of it. But the typical behavior for centering content (`margin: 0 auto;`) does not make any difference compared to left aligned content when the window is smaller than the content. The only hard to judge situation is, when the window is not maximized, but larger than the content and I think for that case it is pretty hard to judge if left aligned or centered content is superior.
Yes, but why make it left aligned?
I like the limited width and left aligned text is also good, but IMHO the content as a whole should be in the center of the web page.
Why should the content be in the center of the webpage?
Because the user is typically looking at the center of a screen.
Sure, you can organize you windows in a way to compensate and center the content manually. But I think there are two usage patterns:
1. maximized browser windows
2. non maximized windows
For the first pattern, the above statement is essential. Naturally, the user is looking at the center of the screen. The second pattern is somewhat unpredictable, because you neither know where the window is on the screen nor the size of it. But the typical behavior for centering content (`margin: 0 auto;`) does not make any difference compared to left aligned content when the window is smaller than the content. The only hard to judge situation is, when the window is not maximized, but larger than the content and I think for that case it is pretty hard to judge if left aligned or centered content is superior.