There's no point in arguing about the technical aspect of this anyways. It's purely a social manipulation campaign by their marketing department, they as much as admit to it in their advertising.
Because it's the right thing to do. They are intentionally degrading the privacy and security of their own users every time an iPhone user opens the Messages app and sends a message to an Android user. They do this because they want a competitive edge, and value that over the privacy and security of their users. They're contributing to the problem of proliferation of various messaging apps, which makes it harder to contact people, and harder to evaluate what method of contact gives the most security and privacy.
I'm not claiming Google is any kind of saint here. They are the definition of message app proliferation (and message app graveyarding). But at least Google, with RCS, is finally trying to do messaging in a way that's open to interoperability.
I'm also not claiming that Apple "must" open things up. As you say, it's their prerogative not to. But I am also free to judge that decision as a net negative for society, and demonize them for it.
Your argument isn’t really very compelling to me, and as another poster mentioned, iMessage may be patent encumbered and not able to be interoperable or offered on other devices. What if Google is choosing not to license the relevant patents to score some points against Apple and take the heat off themselves, as they have potential antitrust cases pending due to their ad sales model.
To me, Google is the monopolist trying to force the issue in their favor. It seems like your argument is that users of Google products deserve the benefit of Apple services without being customers of Apple. Just like Google benefits from ad auctions on their ad sales channels even if they don’t ultimately serve the ads, as they have collaborated with Facebook to not outbid each other. The track record of Apple and Google suggests that while Apple has always been somewhat proprietary, and hasn’t ever suggested otherwise, Google presents itself as open while pulling shenanigans behind the scenes to their own benefit, and their openness is mostly marketing. The existence of projects to remove Google services and difficulty in rooting many Android phones further illustrates this point. If Google were open, we wouldn’t need Microsoft to implement running native Android apps on Windows 11. They’re trying to recreate what Apple already has with the macOS and iOS unified experience, and they’re further trying to erode any advantage Apple has in the market by conflating what kids do to each other with what Apple does in the market and for its users, who buy Apple products because they are Apple products and function the way they do, including iMessage.
I just don’t see how any of this calls for demonizing Apple. Any attempt at interoperability would reduce the security guarantees of iMessage, as they don’t have a Secure Enclave in the way that Apple devices do. Low end Android devices are not the target market for Apple products and services, and neither are high end ones. You say that interoperability would improve the security of messages sent from Apple devices to Google devices, but it’s also likely to lead to bugs and a degraded experience for users who rely on blue bubbles to know that their messages are secure and end-to-end encrypted, and on an app, service and protocol, that they know won’t just be canceled and forgotten about in a year or two.
iMessage on Android dilutes the value of the Apple brand and the Apple experience. You may not agree, and that’s okay. I won’t demonize you.
Are you sure you're not thinking of FaceTime? Apple was planning on opening it, but was forced to change plans due to patents.