Your mobile device doesn't have enough RAM, as that's what mobile devices do when they run out. Try closing everything but the game first.
Different game engines have different requirements. Among Us is Unity, Fortnite is Unreal. Fortnite is also a first-party game from Epic, which means they can directly optimize it for mobile devices. Unity is a third party game engine, and thus the developer may rely on the engine or third party libraries they have no control over.
Though as I said, if the game spontaneously closes, that's usually the OS closing it due to running out of memory, which means it could be leaking memory or something else on the device leaking memory which makes the OS close programs that are running.