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Most Americans have few or zero regular international contacts. The US is geographically huge and there are only two other nations within relatively reach.

Personally I have one person in my contacts who's not in the US. This is someone I communicate with about twice a year, and we do indeed use WhatsApp for that.

For non-North American recipients from a North American phone number, you have to prefix with the country code[0]. So unnecessary if you're in the US and want to call Canada or Jamaica or wherever. But say you want to call someone with a UK phone number, you just send the SMS to +44-<their phone number>.

[0]: https://en.wikipedia.org/wiki/List_of_country_calling_codes

As a leftover artifact from growing up not allowed to call long-distance, I frequently think of things like "Is long-distance texting charges a thing? Will I get charged for texting my friend with a Mexico number? Does it matter if she's currently in Mexico or the US? Does sending a picture incur a fee even if an sms doesn't?"

Social networks.

But as other comments mentioned, the number of people I "text" with regularly without a U.S. phone number / the ability to send and receive SMS is very small.