Well one reason is double spends are incredibly rare, the fear far outweighs the reality. The other reason is most merchants wait for at least one confirmation...
Right, double spends attempts are rare because few merchants are using 0-conf. So if 0-conf became more popular then I'm sure double spend attempts at those places would be much more common.
So I do think 0-conf is needed at brick and mortar shops and protection is needed there especially. Just for peace of mind, businesses need to know that there is some protections for them. Detecting double spends like you mention should be standard and built into all wallets/services.