iMessage is exclusive to iPhones and works in the same application that does SMS, seamlessly.

The problem OP is alluding to is that some people avoid talking with people who don’t have iMessage, because SMS costs money. So there is a bit of peer pressure to use it, and a lot of people are stuck in iOS because of that.

It’s not that different from Signal/Telegram/WhatsApp, those apps are also closed and rely on network effects. But they are free and work on other OSs, while iMessage requires a relatively expensive phone.