In most browsers, the default style presents text adjecent to all sides of the browser window, with no margin. This is a throwback to early days of screen design, when screen real estate was considered so valuable that deliberately wasting it with whitespace was sacrilige.
Of course, in centuries of design on paper, nobody ever put text right up to the margins. Everybody knows it’s ugly and not what the eye wants. Thus, when you see a web page using the default style, which I end up with myself out of laziness, people have a reaction to it as ugly.
Screens are now big enough that it’s time to change the default style to be one that is easier to read. And that means margins. If a page designer wants to put stuff up against the edges, they can easily define their own stylesheets now to do this, so let them do it. I doubt they ever will put text there, though they might put graphics or their own custom margins. If text to the edges is a choice that nobody would make if given the option, it sure seems like silly default to have. It won’t break anything, you can just make the window wider, or make it a user option (which I believe it is in some browsers, but rarely set).
And then more people could use the default for quick pages without having to think about style every time they spit out a web page.