Living in a country where WhatsApp is ubiquitous, I really don't understand that. Is it just a lack of imagination on Facebook's part? Companies, from tiny ones all the way up to enterprises, will pay serious money to be able support customers over WhatsApp in an effective way. And then I've not even begun to consider the marketing channel WhatsApp could be (also without turning into a spam hotbed).
Facebook is the gatekeeper to WhatsApp, effectively a monopoly to a piece of core telecom infrastructure for a lot of countries. They could squeeze this out to the max. Right now, they give all of this away for nearly free and share the profit with the likes of MessageBird and Twilio. TBH i'd prefer they tax businesses than my eyeballs.