Almost all parallelism in game engines is for very specific parts of the engine and almost none of the gameplay stuff is paralellizable. What people who haven't actually had to go through and solve the problems presented in game engines often times misunderstand is that when your game is running poorly because everything is happening on a single thread, almost all of this speed issue is because of rendering. Then physics. These are very hard problems to solve and it's more complicated than "use an ECS" to solve them.
> almost none of the gameplay stuff is paralellizable
Define gameplay. If you have some simulation happening as part of the gameplay, parallelizing it can be quite useful vs killing one CPU core on it. Physics is just one common example, but not everything is about physics. You can simulate whatever.
