In many countries, SMS isn't free (or didn't used to be). That's the reason why Whatsapp was successful in the first place, it was cheaper to use Whatsapp over data than to use SMS for texting.
A single data point: In Haiti, SMS was 1 HTG/message. With 10 HTG, you could activate a plan for the whole day that was more than enough to use WhatsApp for unlimited text and pictures (MMS was not a thing). Now, SMS is only used for carrier ads and 2FA.