Because they only need to be trusted until the transaction is complete. Banks must be trusted in perpetuity. The trust model is much stronger when any breach of

Because they only need to be trusted until the transaction is complete. Banks must be trusted in perpetuity. The trust model is much stronger when any breach of trust can lead to instant reaction, moving all funds away in seconds and blacklisting them. That's why friendlier hardware and software wallets are being developed, and protocols to automate most of the tedious work. Look at for example the payments protocol with the ability to verify the identity of the seller (via TLS certificates), transmit your delivery address and get a digitally signed receipt in one click. With support for extensions it can even do things like silently negotiate for who to use as a trusted arbitrator and the time period for settlement (after X time without it being contested, the seller can withdraw the money without need for interaction). Bitcoin can emulate everything banks do, and still have far stronger security guarantees. Banks simply can not do the same without adopting public cryptographic logs like that of Bitcoin.