You suggest that a site owner should design the page how you want it, not how they want it. But neither is the case. Their task is to design it in a way that is acceptable for many different people. For most users, maximising the browser window does not mean they want text paragraphs to become very wide, it just means they want to block out distractions. And most readers benefit from plenty of whitespace and a conventional line width of 50-70 characters. It’s not that the designer is appalled by your preferences, it’s just that you aren’t the only person who matters.
You say the browser “should” be your agent - but it is. You can use ‘reader’ mode or any number of browser extensions to tailor things for your own tastes, or even copy and paste the article text somewhere else to read it however you want. What you cannot reasonably expect is for every site to be ideal for your personal tastes out of the box.
I guess I just expect the web site to at least not deliberately make itself terrible such that users have to resort to big hammers like extensions, "Disable Styles", "Disable JavaScript" and "Reader Mode".
Web sites are fast, flexible, responsive, readable, scrollable, accessible, and respectful of user preferences... by default, from the moment you add text between <body> tags. Then web developers add code and CSS to make them worse. Sometimes they make it better, but very often it's worse.
It sounds like you prefer that 'academic' style that rests heavily on browser defaults. I personally like that style too, it's not an uncommon preference for HN users. But you must realise it's very niche? Most users of most websites find that kind of design difficult to read and use, so it's rare to see it outside of academic/engineering circles. All you're doing here is asserting your own niche preference, and making out that it's what the majority of people want. It isn't. That's why designers design things the way they do, not (in general) because they are incompetent or malicious. And that's why it's great that you have the option of using extensions/reader-mode/copy-pasting/whatever, so you can effectively redesign anything just for you with a small amount of effort. If you expect everything to already be designed for your own unconventional preferences, at the expense of most other users, then that is unreasonable.
You say the browser “should” be your agent - but it is. You can use ‘reader’ mode or any number of browser extensions to tailor things for your own tastes, or even copy and paste the article text somewhere else to read it however you want. What you cannot reasonably expect is for every site to be ideal for your personal tastes out of the box.