In America, iMessage got there first. Even though it's technically worse than WhatsApp, enough people use it (a majority) that it doesn't matter that it handicaps its own users with SMS and green bubbles. People think it's all the other phones' fault rather than the iPhone's fault. It's a massive driver of iPhone sales, and Apple execs are on the record acknowledging that.
That's not how it's supposed to work. The better messaging platforms should be competitive. Apple stock would tank if they were forced to open up the iMessage API.
If I were on the board of Apple, that's precisely why I'd be pushing for it to be open. I would be terrified if my company's value were in a stupid chatting protocol. It's the difference between accidentally driving off a cliff and purposely driving off a cliff with a parachute. I'd choose the parachute and make money/goodwill from the exposure vs. losing everything because the "blue sheeple" woke up.
They do this with a lot of things, if you try to use a password manager on iOS other than Apple's, it's so restricted it's almost unusable.
Source: recently switched from iPhone to Pixel, not sure how long I can keep it up though
The argument that iMessage is a service provided by Apple for Apple products and that they should be allowed to do whatever with their products is the same argument they've used to restrict App Store, force developers to use their payment systems and charge outrageous cuts of every payment a user might make through there.
iMessage plays quite a notable role in this, as a user whose friends use iMessage (more prevalent in parts of the U.S. I think) might feel unable to move away in order to be connected to their iMessage-using friends, in turn ensuring that they keep buying iPhones and paying Apple a 30% cut of all their app subscriptions from Duolingo to Disney+.