UPI is owned by NPCI(gov entity) and runs on top of IMPS both the networks are strongly regulated by RBI the central bank.

There is fair amount of regulation and KYC/ AML requirements before an app/service gets direct access to the network and even then it can be pretty challenging. WhatsApp struggled for years to get UPI integration .

Holding money in a wallet has even more regulations than merely transferring it .

Payment networks tend to centralize by their nature. I see pix or UPI as competitor to Mastercard or VISA , they have proven it is possible to run a network far cheaper than claimed