There’s no need to be a jerk. For some of us it is just unexpected to hear that folks have phones where they download their primary messaging app. It would be like downloading an app to make calls - it’s just such a fundamental, core feature, that I wouldn’t really think to go third-party for my daily usage. It’s not that it makes no sense, but I hope you can understand why that is a little surprising to some people.
I know that in the 20 years or so since I first got a cellphone I have just used whatever is on the phone natively. Nothing wrong with using WhatsApp, you do you. But that was how it used to be, and WhatsApp did not become so dominant until recently.