I think the real point of irritation here is all the 3rd party junk that gets loaded in that has nothing to do with the main functionality of the site that you are on.
Too many pages obscure content by showing ads, showing annoying popups (no I don't want your newsletter!) and by wasting screen estate on some "float on the top" menu you can't get rid of.
The thing I miss most on my smartphone is NoScript. Luckily Firefox has "reader mode" which helps a lot, but it sometimes doesn't show whole content.
Web browsers are a platform for sure, but the simple content should stay simple.
Developer: "Well, we can either build our website in document mode or in application mode."
Management: "What's the difference?"
Developer: "If we build it in document mode, it loads quickly and cleanly and..."
Management: "Does document mode let us put the modal popover window that got 5% of our visitors to join our newsletter last month?"
Developer: "Well, nope."
Management: "And the third party tracker?"
Developer: "Again, no."
Management: "So we can't monetize or track how many sticky eyeballs are consuming our content?"
Developer: "Nope."
Management: "App mode it is."
Developer: "But it isn't an app. It's just static pages."
Management: "APP APP APP APP APP APP. I hear you can go on a course to build apps in less than a week."
Developer: starts checking his LinkedIn for an employer who doesn't make him want to take a pickaxe to his own face
First, while I agree with this being the biggest issue, there is a little bit of nuance to your other point.
> If you believe that the browser should only be a document reader and not an application platform then go ahead
The thing is, the overwhelming majority of those applications are glorified document readers, with a bit of social media plug ins thrown in. It features rich media perhaps, but you don't need JavaScript for an image or video tag. Of the remaining applications, the majority of use-cases is forms to send over the web. This can often be enhanced by JavaScript, but is also not that insane in terms of needs. Then we get to the remaining use-cases of the "application platform".
There's also the part where most websites are just plain badly designed. For example, many sites that have a video that auto-plays when you open it, even if you open a text article and the video is totally unrelated to what you were watching. Worse, some of them even tend to autoplay to a new video, often redirecting your webpage, interrupting the reading experience. Sure, I can pause. It still downloads and buffers a video I never intend to watch. Similarly, whatever happened to progressive enhancement? Websites that give you a blank page unless you download three megabytes of JavaScript. WTH.
In one word: Frameworks.
In more words: Building HTML in two different places is typically not very DRY (Don't Repeat Yourself) and if you are assuming a lot of very dynamic content that want to update on the fly from the JS side of things all the way down to micromanaging the HTML generation at run time, well then all of your HTML building moves into JS to keep things DRY.
The hope for the future is so-called "Universal" JS (which is interesting in that its kind of circling back to the origins of JS (LiveScript Server) and ASP Classic): now that we have a somewhat standardized JS platform on the server side (NodeJS) we can use the same JS to build HTML on the server or in the client...