Web browsers have become to Javascript what Emacs is to Elisp. It is an interactive, extensible, "everything application". The main difference is that right now, just about everyone with a computer uses a web browser.

Personally I prefer to use a separate, dedicated service to run my language models, and I bounce between a few clients for various things. One of them is a (local) web interface (open-webui).

If my data is already in the browser (Sheets, Gmail, other websites to scrape) why have another app instead of doing the work in the browser?

Why does the address bar go to a search engine?

When you use an LLM, you provide it context. Where do you usually get that context from? Some service, JIRA, whatever, in your browser.

For the same reason that a netbook/Chromebook is probably enough for most people: users rarely install native apps on laptop form factors these days - apart from the "essentials" like an office suite, and possibly specific programs they use to get work done (image/video editors etc.).

A browser is one thing that users will go out of their way to replace, making it a pretty attractive vehicle for stuff like this.

You missed the joke