I mean you can install Google messaging apps on iOS (not that I would want to use them…). But try that the other way around. Apples option to not using SMS would simply be to provide iMessage for Android. Problem solved. They would very likely become the main messaging platform by doing so. Currently the majority of the market is likely split between WhatsApp, Telegram and WeChat.
But obviously they fear that this would hurt iPhone sales. At the same time this also leads to iMessage being irrelevant in the majority of markets where iPhone isn’t as dominating as in the US.