Because it is a smaller download, is faster to render, easier for accessibility tools, and is more likely to work in readers and browsers of all kinds.

Yeah, a single `body { max-width: 50em }` is not going to change any of that, and actually makes it more accessible to a wider audience. The entire point of typography and formatting is to make text more accessible. Lack of layout is the antithesis of that.

Then there's the separate issue of overlong paragraphs, which is simply a sign of poor writing (again making the text less accessible), unless you're trying to argue that the use of fewer <p> tags makes the page faster to load and render?

Now, I do wish that browsers had saner default styles, so one wouldn't need even that single line of CSS, but that's not the world we live in, and for whatever backwards-compatibility reason we're stuck with how things were in 1995.

> body { max-width: 50em }

I have this 16:9 screen. I'd like to use all of it, thanks.

Filling a wide screen with one column is just going to hurt your ability to read it. That's not a great desire to have.

(I'd suggest more than 50 but not a whole lot more, not nearly enough to fill a normal size screen.)

Now you’re agreeing with the main idea but you wish the default was more to your preference. Read his own words about this here: https://news.ycombinator.com/item?id=39731878

I don’t understand though. Times new Roman and clones of it are very readable fonts. It just doesn’t look like short form marketing material.

If you defend the styling by saying it's efficient, and they say you can have efficient style so that's not a good reason, they are not agreeing with your main idea.