There's a difference between the two, right? The browser is not just "rendering polygons" - it's actually laying them out. Position of one affects another.
Have thousands of physics entities in a game interacting with each other and see how it goes. That's why we see things like "millions of falling ducks" as an impressive demo or benchmark for 3D programs/engines.
See, when you're talking about rendering, you're saying millions. And even then you're talking about millions of elements in a physical simulation, all interacting with each other etc.
The browser struggles to display even a few hundred elements. You can't even animate something efficiently if that touches even the smallest part of a layout.
So yes. There's a difference between the two. The browser at the core is a system for rendering a page of text and a few images. It's unbelievably inefficient for almost literally everything else.
Have thousands of physics entities in a game interacting with each other and see how it goes. That's why we see things like "millions of falling ducks" as an impressive demo or benchmark for 3D programs/engines.