> Why does a browser need a revenue model?

Because development isn't free.

> Why can't a browser just be a piece of software that's "finished", with the occasional security patch?

Developing security patches for the dwindling user base such a browser would have isn't free, either.

But even if such a browser existed, the browsers people would actually be using would be the ones evolving with the things people want from the web.

Unfortunately keeping modern software secure is a full-time job. Internet browsers are just one giant attack surface. This is especially complicated by the speed at which the implementation of new features is required to keep a general audience interested. Unfortunately, a user like my mother wouldn't care how ideologically well-meaning a browser was if she couldn't use it for watching online media. I recently went looking for a lightweight browser for an older linux box, and it was hard to find anything that didn't either bundle Chromium or just forego having Javascript at all. Keeping up with modern web standards is pretty much an impossible job for a team of hobby developers on the basis of features alone ( WebGL, WebAssembly, DRM etc ), let alone security.

A good chunk of linux and related development is done by programmers being paid by some for-profit company in whose interest is it either to have those chunks developed or who want to keep their employees happy by letting them do open source contributions. Without this invisible injection of funds, linux development would slow down significantly and it might not remain competitive.

Browsers are a lot less modular than operating systems and they operate in a different market (eg. contributing to their development doesn't help the bottom line of any company particularly), so Firefox doesn't get a lot of paid for third-party developer time. Hence, Mozilla makes unfortunate compromises.

Torvalds is employed by the Linux Foundation.