Games are simulations, arguably the hardest paradigm of software. Games have always included user interfaces, and in many cases they are just user interfaces. Web apps for the most part are also just user interfaces. React has basically adopted the equivalent of a game loop architecture. I guess the only thing holding back web apps becoming basically game engines are search engines like Google indexing different parts of it. In other words, search engines are better adapted to index and make sense of files on a computer, rather than a runtime simulation of some kind.