Because that way people thought they were directly paying for the service they were using, instead of being the product of said platform, having their personal data harvested and sold to the highest bidder?

Are you saying that people perceived WhatsApp as better than SMS or better than Facebook?

As it happened, both.

The red flag is to look at a free meal and not wonder what the catch might be. Especially to this day, with all we learned about what the tech majors do with all the data.

That's not my point. My point is why would the majority of the world do this when they knew it was going to be paid.

Back then, the norm was to pay for a service. When it's good and the price is fair, people use it, especially when the alternative was feature-limited SMS paid by the message at inadequately high cost. And Facebook isn't free: you trade privacy and exposure to customized ads in exchange for access to the service, so your comparison is biased.