It’s because back when smartphones and Whatsapp were new, unlimited text messaging plans were either expensive or unavailable in much of Europe (and I would imagine other places as well). From my understanding these kinds of plans were much more common in America.

When your cellphone plan has limited text messages, but sending messages via Whatsapp takes so little data that it might as well be unlimited, the barrier to early adoption becomes very low. So people start using Whatsapp, and get their friends to use Whatsapp. And once that ball is rolling it becomes very hard to stop.

These days people use Whatsapp because everyone else uses Whatsapp.
It’s the assumed default.

Edit: Heck… even to this day I have limited text messages.
My current cellphone plan is for 12 GB, Unlimited calls, and 500 texts.

And I’ve not sent a single text message in months, if not years.