WhatsApp had implemented the feature as early as 2018 but they were denied permission to launch. They were finally granted permission in 2020 … but only to onboard 1 million users. This limit was increased to 100 million in late 2022 and then removed in late 2024. (
https://coingeek.com/whatsapp-pay-to-expand-upi-services-to-...).
WhatsApp currently has 600 million active users in India and has been the most popular app for a long time. If it had been granted permission in 2018 it would be the most popular UPI app now. There wouldn’t be a competitive app ecosystem like there is now.