This is probably a better comparison than one might at first think. A game engine has a second, hidden, part of the iceberg: The production pipeline for all th

This is probably a better comparison than one might at first think. A game engine has a second, hidden, part of the iceberg: The production pipeline for all the 3D world/art assets. That's actually where most of the work goes -- hundreds of artists go into a AAA game, and all the art they produce must be technically excellent along a bunch of dimensions that have nothing to do with what it looks like (and in fact, generally make it harder to achieve the look you want.) Game engines that can wire physics simulation to 3D rendering to window abstractions are easy-ish to make, similar to how web UI frameworks are easy-ish to make. All the production pipeline stuff is where the real meat is, similar to how in WebUI and, package management and transpiling and bundling and deploying is a hard problem. I think the game side has it even harder than the web side, though, although they usually have the benefit of only needing to work on a defined set of platforms, rather than in "the" browser.