(Samurai wallet is defunct, but the principle holds up)
Sure you can transfer the private key from one device to another, but (a) you can't know the other person didn't retain a copy of it and (b) you would be limited to spending the exact amount you have in an existing transaction because you couldn't send a transaction to the chain that splits it.
Transacting bitcoin private keys without going to the network and trusting the other party to not scam you defeats the whole purpose.