Web browsers have three purposes: document viewer, remote paperwork machine, and cross-platform application framework. I could throw together a browser fully capable of the first two in a month. (Much less time, if you're okay shipping a prototype, which personally I'm not.) Bank websites are not complex, unless you count the business logic: there's no reason they shouldn't work in Dillo.
This would simply not be possible, if basic web browsers were more popular. The React / Next.js app wouldn't run, and then the users would complain, and the Frontend People would try to say "just use the WHATWG VM", and then the Enterprise People would say "we've blocked that so people can't play video games at work: fix your end" (ignoring that they only blocked programs named chrome.exe, and everyone's got Firefox or a renamed Chrome installed), and then the Frontend People would have to fix it because that's what the client demands. (For B2B, at least, but B2B is lucrative enough that best-practices proliferate.)
It's a chicken-and-egg problem. The harmful consequences of Postel's principle are well-known, but the solutions are also well-known. We only need to get past a certain threshold before the problem's effectively solved, and that can be done by starting in a particular domain and working our way outwards.
Yes, almost any competent developer could define a simple document format that would make it easy to write viewers for all the niche OSes, but he cannot persuade content creators to adopt the simple format because most creators (writers) don't see anything wrong with publishing on the web and don't see anything wrong with relying on intermediaries like Wordpress, Medium and Substack to help them publish on the web. Those intermediaries do not want to switch to a simple document format because the user's being dependent on a mainstream browser helps them to track users and to monetize.
So 99% of the time, a user ends up needing the full complexity of a mainstream browser just to read a static document.
> document viewer, remote paperwork machine, and cross-platform application framework
Can I get a show of hands for anyone else who has had multiple jobs where The Frontend People have decided that in order to show something that can only honestly be defined as "a basic document" or complete a few simple <form> tags, the tooling necessary for the job is a React or Next.js app over 1,000 NPM dependencies, and fully reimplementing all built-in functionality from scratch in JS? The Web is simply the land of excess. Nothing is too simple to be overengineered poorly in JavaScript.