Graphical web browsers were originally made, like other graphical programs, for a screen with a resolution of (approximately, depending on hardware) 1200×900, where “maximizing” windows was not really a thing, since multi-tasking was what everyone did as a matter of course, with multiple programs open at the same time in their own window. Web sites were designed to fit in a comfortable window size of, again approximately, 600 pixels in width. Most PC’s were, at the same time, usually equipped with 640×480 screens, with only some PC’s having higher, so to see the whole web site on a PC you would naturally maximize the web browser program to the whole screen. Since multi-tasking was
not really a feasible thing on PC’s back then (due to memory and CPU limitations), it made sense to give the whole screen to a single program, and PC windowing systems afforded this use.
However, when PC’s gained higher normal resolutions (800×600, then 1024×768, and higher) and multi-tasking became more realistic, this did not change the widespread user practice of full-screening programs on Windows! Instead, web site authors simply made web sites wider and wider. The users of large graphical displays did not initially complain, since their screens were wide enough to fit the ever-larger web sites, and these hardware platforms were falling out of use anyway.
And so we come to today, when we have more pixels than we know what to do with, and enough RAM and CPU to run umpteen programs at the same time, but web sites are still designed to be viewed in a maximized window, and users are still maximizing the web browser, and both web browsers and most operating systems are affording this use.