I am also surprised that WhatsApp doesn't have any market share in UPI. WhatsApp analysed that people share gpay/paytm screenshots after payment, so why not integrate payment within WhatsApp but people are habituated with established players and not want to move to WhatsApp payment. Also the main focus of those apps are payment while for WhatsApp it's messaging. WhatsApp doesn't provide any incentive for payment while those apps offer vouchers or shi**y cashback atleast.