The next best reason would be “because they’ve been legally compelled to do do”, but we’re not there yet.
I'm looking at years worth of conversations in Messages right now, with a healthy mix of iPhone-only conversations and mixed-platform conversations. How would my user experience be improved?
The good news is that Google now supports reactions since they fall back to plain text.
https://www.macworld.com/article/610908/google-messages-andr...
A random Google Product Manager representing both sides seems not the way to go.
I mean if you're going to dictate a standard to a duopoly, you need both to agree. I haven't seen the opposite side of this discussion anywhere yet.
Besides, the whole iMessage system is highly tied into the whole iCloud ecosystem, and I don't think either party wants Android users to have to create iCloud accounts to use the messaging app.
Disliking the client apps themselves is of course perfectly reasonable, as some of them are annoying and/or have weird UI that seems to optimise for things other than "I'm talking to another person, not looking to use whatever your pet UI feature is"