Modern webbrowsers are hugely more complicated to implement than a kernel with no user space and very limited driver support (as Linux was back when it was a single person operation).
In fact modern webbrowsers are bordering on being an operating system in their own right. They're already a virtual machine, with their own run times, plus one of the most complicated document markup standards known to man (and to make matters worse, it's not just as simple as supporting correct standards, they have to support incorrect usages of those standards too because webdevelopers often use deprecated standards, tags or even just use those tags wrongly and people still expect those sites to render).
Lets not forget that JS isn't just a simple interpreter like in the 90s. There's 3D rendering with WebGL, audio rendering with WebMidi, real time communications with WebRTC, location services, WASM, and so on and so forth.
Let's also not forget that HTML, CSS, JS, etc are not the only standards a browser needs to support, there's GIF, JPEG, SVG, video formats and audio formats too. There are network protocols like HTTP, FTP, SSL/TLS. And HTTP is non-trivial these days because there's multiple revisions of it, each different significantly from the previous. There's websockets, binary encodings, MIME types, etc.
And all of that isn't even touching on the UI. Even just supporting a modern multi-platform native desktop UI is more than a single persons job. And that's before any of the complexities mentioned above.