Depends on what you mean by "browsers". Take the V8 JavaScript engine for example, it's used in Chrome but can also run stand-alone (I think node.js uses it).

Depends on what you mean by "browsers". Take the V8 JavaScript engine for example, it's used in Chrome but can also run stand-alone (I think node.js uses it). I'm not involved in this space at all but I would guess that the main problem with writing browsers isn't that they're all obfuscated monoliths, but that the amount of work to create an entire browser composed of solidly interconnected parts is still a huge amount of work, because of exactly what you said, it's almost an operating system, at least when you look like fully featured browsers like Firefox and Chrome.