Because it is 2018 and people have forgotten how to static HTML. All that lovely rendering code in your browser goes unused as people rewrite it in Javascript, badly; or even in C++ and then compile to web assembly, that’s all the rage now, because why stick with one layer of abstraction when you can involve three? Oh, and of course everyone rolls their own, so that website with the dancing pigs not only has its own…
Of course this wouldn't be a 'web browser' in the traditional sense, but a (reasonably) safe runtime platform for untrusted code without a central 'Big Brother' who decides what is good or bad for users.