The fact is that a single person with a decent amount of knowledge can write a game engine that is more or less complete, while not even FANG companies can write a web browser from scratch should definitely be proof that the latter is more complex.
Some physics and linear algebra, while I’m not saying is easy, but it is not a complex layouting and CSS engine, with a state of the art language runtime, with all the possible requests, sandboxing, etc. — of course you don’t necessarily have to write an optimized browser, but still, just implementing a usable subset of the web is ridiculously hard.
