Not just Americans. Nobody I know in Britain uses it. Maybe for workplace groups. WhatsApp is most popular in Africa and parts of Asia.
Most people in the USA and Britain have all inclusive plans, so texting is 'free'. Also people are concerned about tracking, privacy etc. E.g. I use Signal for the privacy.
When in Africa. I find it irritating that so much is done through WhatsApp. The app reminds me of criminals and dodgy deals.😅