Raise your hand if just running your web browser has pegged your top-end, multiprocessor, high-mem system in the last month. Both Firefox and Chrome have for me.
I think that's largely due to JavaScript and its ecosystem of abstraction-bloat that has been mentioned in another comment here, along with the trend of "appifying" what should really be static sites. A static page that contains even dozens of MB of content won't stress a browser as much as a "web app" containing only a few hundred KB of countless JavaScript frameworks glued together --- despite the latter presenting a fraction of the actual content.
