I’ve always wondered why was it locked only to merchants, anyone knows?
Apple doesn’t process the transaction, a different payment processor does, Apple just provides the API for developers from said payment processors to implement in their app.
The current closest thing for tap to pay for consumer to consumer payments is NameDrop then using Apple Pay Cash through iMessage however this is a less seamless method and is only available in the US. I can however see Apple expanding the tap to airdrop features to Apple Pay Cash in the future, perhaps with iOS 18.