"They can afford it" is a terrible argument. There's literally no upside for Apple providing their infrastructure for free to third parties, particularly given that it's a potential vector for flooding their customers with spam.

Up until quite recently, most phone carriers metered the number of texts you could send per month and then charged extra. Many still charge per text when you're roaming overseas. Perhaps Apple could offer API access on commercial terms to third parties but that's their decision.

I really don't think anybody should think about their decision, they are too big for fully owning the platform.

Additionally, iMessage is full of scams and spams already, it's not hard to buy a box of old iphones and turn them into spam relays and that's exactly what is happening now.