I guess it's similar to how people use iMessage because it's the pre-installed messaging app. Both fall back to of SMS/MMS if the other party isn't using the same closed ecosystem. I.e. Google's proprietary end-to-end encryption or iMessage.
Maybe if I'm lucky they'll briefly remember they have a website for sending messages and they'll finally update it so you can react with more than seVEN EMOJIS! It's been YEARS!!!