"Browsers should be simple enough to be implemented by a single person in a week."
They are. It all depends on who defines what a "browser" is and what it can do.
The browser I use as a "daily driver" is a 1.3M static binary. It is 5.7M of C code, including graphics stuff I do not utilise. It compiles in under a minute on a low-end computer.
This browser allows me to "browse" hypertext. It displays HTML tables beautifully in textmode. I can download files. I can save HTML as formatted text. It supports FTP. The program has various settings for cache size, character set and so forth. I can script the browser using tmux.
This browser is all I really need.^1 I read HTML then decide if I want to download or save something. Simple. What else is there, really. When I want to run programs I do not do that using this browser. I have separate compilers and interpeters for that. When I want to listen to audio or watch video, I do not try to listen or watch using this browser. I have separate programs for that. And so on. I do not try to live inside a single pogram. In every case of a non HTML-reading/saving or www/ftp browsing task, the separate programs do a better job than the browser could. If this sounds something like the "UNIX principle" maybe that is not a coincidence.
Sure, we can take each of those individual programs and integrate them into a single program, and still call it a "browser" but why would anyone want to do that.^2 What is that concept in programming called "separation of concerns". Someone once called it "an ordering of one's thoughts". When people use browsers written by web advertising-supported organisations, then the "organisation of thoughts" is going to revolve around advertising (requisite tracking and personal data/metadata collection) and how to integrate it into the browser. When I use the text-only browser I choose, the ordering of thoughts is quite different. I see no advertising. Ever. I still consume the same amount of "content", maybe even more because this 1.3M browser is quite fast.
1. Major corporations operating on the web today, e.g., banks, can obviously force people to use certain browsers for business and important personal matters. For recreational web use, however, people can choose any browser.
2. The answer is not "For everyone's convenience and free enjoyment." That is why we have people trying to "un-google".