They control the OS so they can combine and feed all your digital information to an LLM in a seamless way. However, in the very long term, I think their advantage will go away because at some point, LLMs could get so good that you don't need an OS like iOS anymore. An LLM could simply become standalone - and function without a traditional OS.
Therefore, I think the advantage for iOS, Android, Windows will increase in the next few years, but less powerful after that.