LOL, whilst I won't necessarily include all Android devices in this, I'd argue that iPhones and better Android setups are likely the Most secure device that most people own.
Traditional PC security is way too complex for non-technical people to manage effectively, so the approach of walled gardens and vendor managed security is the way to go.
Not to say that a technical person can't achieve a better level of security with a PC/Mac, but for non-technical users, it's going to be mobile all the way.
It being an app, downloaded from a moderated app store (albeit not a very well moderated one) rules out an entire class of attacks (phishing), which make up a large portion of online banking attacks.
This alone is probably a decent competitive edge over traditional banks.
Traditional PC security is way too complex for non-technical people to manage effectively, so the approach of walled gardens and vendor managed security is the way to go.
Not to say that a technical person can't achieve a better level of security with a PC/Mac, but for non-technical users, it's going to be mobile all the way.