SMS/MMS is still (sadly) quite common in the US and on iPhones that means using the built-in Messages app. So if you want a non-SMS/MMS that's also widely used in the US you get it "for free" by using iMessages from the same application if you're an iPhone user. And this chat also works with your non-iPhone contacts because it switches to SMS/MMS for them.

For whatever reason WhatsApp, Signal, Telegram, and others never won out on Android in the US so the Android users didn't bring along their iPhone using network to those clients either (in bulk).

Google of course has lost out because of their own foolishness: https://arstechnica.com/gadgets/2021/08/a-decade-and-a-half-...