Where do you think all that code lives, then? The support for dark mode, the ability to look at the pixel data of an image so that you can display text with the right color on top of it to have high enough contrast, the subtle animation when you tab between elements in a scrollable grid, the annotations for accessibility, the code that makes drag and drop work like you expect... As much as you don't think it is "hundreds of thousands of lines of code", it really is.
I started a business online five years ago. I used as little JS as possible: the first version was probably 500 lines total. Today, it's approaching a quarter million, and it's all just me. Why? Because I want forms to show helpful errors when you type the wrong thing, and my pages to show appropriate visual cues at the right time in the right places, and for the page to look right when dark mode kicks in on your laptop or you resize your window, and for decimals to be formatted correctly for folks in Europe. Or for the power users who expect things to be fast, so navigating forward and back loads and caches data correctly. Or for folks with high-dpi displays (or as it were, folks who zoom in) who don't want pixelated icons.
And has it paid off? Yeah. As an example, my service hosts a surprising majority of blind podcasters, including the American Council for the Blind. I didn't get here with a spring in my step and a strong belief in semantic HTML, I got here by investing in all the fiddly edge cases so no matter what size your screen is, the browser you use, the language you speak, the currency you want to get paid with, your ability to use a mouse, your screen reader or level of vision, you still have a great experience. As much as you'd like to call it a fad or accidental complexity, it's really just your inexperience with actually building good user interfaces in the 21st century.