I'm using either a 27" 1440p monitor or the built-in screen of my 15" macbook. Either way, modern web makes fonts and UI elements a tad bigger than they have to be, and there's usually too much area wasted on whitespace between them.

Desktop UIs are meant to be compact because mice and trackpads allow nearly pixel-level positioning. Mobile UIs, on the other hand, have to have large elements and ample spacing between them because tapping stuff with your fingers is inherently imprecise. You can't really have both these things at the same time.

Out of websites that are currently live and have good desktop versions, I'd point out HN and reddit (the old design).

And then there's Facebook who has separate mobile and desktop websites but recently chose to redesign its desktop one to look very mobile... I can't imagine the justification behind this one.

You're certainly welcome to prefer more compact UIs on your very large screens, but it's only that: a preference. There are many reasons why other people either prefer or need (i.e. accessibility, touch) larger text and UI elements to be able to use these websites.

I personally prefer (and may well someday need) larger text and UI elements, because my vision and motor skills have gradually declined. I also prefer (and soft-need) lower information density because higher density is a cognitive challenge (ADHD).