[nikcub]

It isn't just if you step away from a computer. A session token that never expires is as good as a password, but with weaker protection.

If I intercept a session token via a proxy, network dump, XSS or browser bug I can use it and replay it at any time in the form it was intercepted.

Passwords get sent once and are usually protected and encrypted or hashed on the server. Session tokens are not, hence why they need to be temporary.

[gojomo]

Holding everything else constant, shorter session tokens reduce one avenue of exploitation, yes.

But everything else isn't constant: shorter sessions mean more password-typing-transactions, and especially into older tabs that have a "logout successful for your protection" message. That increases the risk of a successful phish, including by the same vulnerabilities you fear could compromise a session token. And practically, the problem with a password compromise is that it gives access to a indefinite stream of new session tokens.

So there's a balance between session-token-risks and login-transaction-risks. I doubt 15 minutes is the optimal tradeoff time -- I'm sure it isn't for me, with my habits on my own computers, and I haven't seen any rigorous evidence it's the right level for the banking masses. Its maddening uniformity across the industry "smells like" an arbitrary check-box from some regulatory document somewhere.