iMessage is all about convenience. Sharing of media is much easier using imessage. Most in the US don't use Whatsapp. Few care to mess with Facebook messenger. They just go to the default texting app by, well, default I suppose. Most of the time that results in compressed images and compressed grainy videos. That really is the only allure to me. I know to use Google Photos or Dropbox links but most don't care to mess with that. My wife for instance couldn't figure out how to save a video off a link I sent via Google Photos. It isnt exactly intuitive. So with iMessage, it's mostly the cliche that's always been associated with the iphone. "It just works". iMessage is about the only thing missing from my S21 Ultra. That and a working DJI app for my drone, but that's the app dev's fault.