After the fall of mt. gox there was a huge push to get exchanges to do this.
My guess is that the costs of doing this (security is quite hard: anything that can sign messages using cold storage keys can obviously also empty the cold storage, since transactions are simply signed messages) does not outweigh the demand for this. Most traders don't care (and subsequently get burned).
> And the transparent bank would publish a daily record of balances for each account
I think this is a terrible idea though. Even if done anonymous it gives way too much information about a market and its participants. IMO just use a merkle tree so everyone can check their own balance.
Anyone with a lot of AUM looking to invest would beg to differ - or well, they might use the information while trading on a different exchange.
Also draws a nice crosshair on your exchange, so that hackers can exactly calculate cost/benefits as well as learn from your systems (how you divide your cold storage over different addresses, how that changes over time, how money from different dummy accounts effects cold storage in what way).
Tether published an audit a month ago.