Centered 1000px Wide Page Layout
The smallest monitors in use these days are at least 1024 pixels wide.
Thus, a 1000px wide content area will fit on everyone's monitor.
That's important, because in most cases you dont wan't people to have to scroll sideways to see page content.
Monitors were not always that wide.
Mid-late 1990's - About a 600px wide layout was the standard (had to fit on 640x480 monitor).
Early 2000's - About a 750px wide layout was the standard (had to fit on 800x600 monitor).
The design standard for fixed width pages has remained at about 1000 pixels wide since about 2010, or perhaps a bit earlier.
As monitors continue to improve, building pages with 1200px wide content is becoming more common.
Here is some
information about screen sizes and examples of old Web page layout designs that were influenced by width limitations of old computer screens.