Basically, there are basically (not exhaustively) 2 kinds of programs one can run in a computer nowadays:
1.1. A program that is installed, permanent, has direct access to the Operating System, can draw whatever it wants, modify files, interact with other programs and so on; 1.2. A program that is transient, fetched from someone else's server at run time, interpreted, rendered and executed by another program that bridges the access of that transient program to the OS and other things.
Meanwhile, web browsers have basically (not exhaustively) two use cases:
2.1. Display text, pictures, videos hosted on someone else's computer; 2.2. Execute incredibly complex programs that are fetched at run time, executed and so on -- you get it, it's the same 1.2.
These two use cases for browsers are at big odds with one another. While stretching itsel f to become more and more a platform for programs that can do basically anything (in the 1.1 sense) they are still restricted to being an 1.2 platform. At the same time, websites that were supposed to be on 2.1 sometimes get confused and start acting as if they were 2.2 -- and other confusing mixed up stuff.
I could go hours in philosophical inquiries on the nature of browsers, how rewriting everything in JavaScript is not healthy or where everything went wrong, but I think other people have done this already.
One thing that bothers me a lot, though, is that computers can do a lot of things, and with the internet and in the current state of the technology it's fairly easy to implement tools that would help in many aspects of human existence and provide high-quality, useful programs, with the help of a server to coordinate access, store data, authenticate users and so on many things are possible. However, due to the nature of UI in the browser, it's very hard to get any useful tool to users.
Writing a UI, even the most basic UI imaginable (some text input boxes and some buttons, or a table) can take a long time, always more than the time necessary to code the actual core features of whatever program is being developed -- and that is considering that the person capable of writing interesting programs that do the functionality in the backend are also capable of interacting with JavaScript and the giant amount of frameworks, transpilers, styling stuff, CSS, the fact that all this is built on top of HTML and so on.
This is not good.