Why do websites not use entire width of browser?

What reasons might a website not use the entire width of the browser, like Gmail does?

For example, StackOverflow does not use the entire width — it has left and right margins. This means the entire contents of a page are presented in a column, with white space on the left and right.

Is it a technical reason in most cases, such as to accommodate the display in all types of browsers, or are there design reasons for this decision?