Because they don't make money directly from iMessage, they make money by selling the hardware, and the fact that iMessage only works on their hardware increases it's value and further locks you into their ecosystem.
Lock in - how many people are locked into Apple because of iMessage? I think it's just an ancillary service to most people, with WhatsApp, Messenger, Telegram, or WeChat in China being their main IM app. The longer this is the case, the less relevant iMessage becomes.
I see a lot of upside and not a lot of downside for Apple here