To a certain extent, it's your own fault. (I know, real persuasive with an opener like that...) Maybe it doesn't matter so much once you're used to it, but the point is readability. Extremely wide paragraphs are hard to read. Someone not used to it, will find it slower to read and more prone to error.
Personally, I normally use a browser window around 1000px wide, though have been sitting at 1200 for a while as too many "desktop version" sites demand there be no inbetween.
If your habit is a maximized window on HD (or bigger!) screen, well that's your problem -- or it should be, depending on the website, whether they've found a way to use up all that space.
Also if you're using ad blockers, that may leave a lot of white space... should be N/A on eBay, but it really gives you some idea of how much sheer trash some sites blast you with, or try to.
Best practice is for a website to be readable at any size, "responsive" as they say -- the big difference is that, since the 90s left, no one had a 640x480 monitor anymore, and everyone got comfortable with say 1024x768 being the smallest supported. Then everyone started walking around with 480x720 and such screens in their pocket. And pixel scale is all over the place, we have 2k screens in handheld sizes; we never used to have to worry about scaling, maybe a bit with laptops but we squinted and managed -- now all that has to be handled by the OS and browser, with only a little input from the web page itself (most importantly the <meta name=viewport content="width=device-width, initial-scale=1"> tag). The rest is handled by some combination of CSS and JS (some responsive changes can be done in CSS, hiding or stacking sidebars; JS can rearrange the whole document, change zoom, etc.).
So, the situation isn't great on either end. Web pages suck, sometimes tailored for smaller screens, sometimes larger, or both, or inbetween as well; clients often have no choice (small screens), or make choices that aren't the greatest (maximized windows on huge screens, using scaling or not).
Tim