Apple’s ideology behind not expanding iMessage to other platforms has been - at least in part - due to the security of the iMessage platform and how it authorizes senders and recipients (like many encrypted services on Apple devices, tokens are encrypted/decrypted in the Secure Enclave on the SoC). Apparently, Apple has low confidence in the diaspora of Android devices and just decided to forget even trying to create a client for Android it could tie down to hardware authentication due to not having a reliable hardware base. This was many years ago.

I don’t know if this is still true or even necessary today, or if they’ve even bothered to explore it recently, but that’s Apple’s main issue. Sure, it also benefits them in other ways such as driving users to their platforms, but this is their main issue.