Twilio's bread and butter is telephony developer relations. Why would WhatsApp get into this market when they can outsource it for free to Twilio who has the product and documentation nailed. The support however is a different story. Once the demand is high enough, WhatsApp are free to revisit. It also keeps WhatsApp low (relatively speaking) cost to run.
The type of messages that can be sent over WhatsApp via Twilio are so restricted and expensive that it was not worth integrating when we looked at this at my work.
Even with Verizon's proposed A2P charge per SMS, it is half the cost to use SMS for conversing with customers compared to WhatsApp. Said surcharge has been postponed repeatedly, meaning SMS is an order of magnitude cheaper to use than WhatsApp
