I buy almost everything with a piece of plastic that represents a company who's agreed to lend me money that represents absolutely nothing except the common agreement that it's valuable.
Yes and credit cards are a learned behavior, not an instinctual thing - and I think not buying an app for $1 is largely based on instinct.
People instinctively or factually know that there are other apps that do basically the same thing for free.
It's the case for messaging apps and for almost any other kind of app. It's hard to beat the price point of a free app, even if it might include tracking, advertising, spying inside their package.
If WhatsApp would start asking for money hundreds of millions of people would switch to something else in a few days, even to a free app created overnight to capitalize on the opportunity.
On the other hand paying for service is the oldest profession going
The idea of trading something valuable for an abstract piece of software or paper is still not really natural to us, and is a learned behavior.