Chargebacks?
That just shifts the scam potential from the receiver to the payer, and makes it more confusing potentially increasing the number of scams. Not to mention the fact that its a lot easier for a receiver to be 'trusted' rather than require the trust of every single sender ever.
Unless of course you're in favor of chargebacks verified by a central authority in which case (1) we can already do that its called an escrow and (2) it can't be built into the Bitcoin protocol itself because it would give one particular escrow an unfair advantage over all others.