The vast majority of people in the US don’t use a 3rd party app for messaging. Why this is I can only speculate. My guess is that we were marketed SMS as a “killer feature” of phones and phone plans during the pre-internet days of cell phones
Here in Europe, SMS used to be so expensive that nobody used them anymore when alternatives such as WhatsApp came up. In the meantime unlimited SMS are included in most contracts at no extra cost (or no choice) and still very few people use it anymore.
Maybe in your corner of Europe. Must be a different one than mine.