Because they're demand deposits roughly equivalent to savings accounts. You don't get paid interest and you don't have any control over how they're invested.
But Mt.Gox is not a bank, and doesn't solicit deposits. They are an exchange. People don't have to store their funds there. If they want interest for their money, or don't want it invested don't store funds there. The only reason people store funds there is because it's convenient and (currently) free.
A proper Bitcoin Bank, on the other hand, is different. Such an entity is designed to store funds. Accordingly, they should stipulate in clear terms to their customers what their liability is for stored funds. If none, that's fine, but say so clearly. Otherwise, say how responsibility plays out and charge account fees if necessary to cover it all. Clients would gladly pay it.
I wish that last part were true, but it's just not.
Why would you think not?
http://money.cnn.com/2011/09/26/pf/bank_fees/index.htmThe ultimate responsibility of someone's money lies with them and what they do with it.
Certainly I agree with that sentiment, but usually people who say that are expecting people to draw false conclusions from it. It is not true that you can operate an eWallet service, put "we are not responsible for anything" in your terms of service, and then you're not responsible if your negligence causes the loss of other people's deposits. Bitcoins are pretty useless if everyone's best advice is basically, "there's nothing safe you can do with them, no safe way to buy them, no safe way to sell them, no safe way to store them", and if you pick the best options that currently exist to the best of your knowledge and following the advice of recognize experts, if anything goes wrong, it's still your fault.
People need to learn to take responsibility for their own actions and stop expecting their hand to be held. Bitcoins are not useless because of scammers and theft. Bitcoins allow people to send millions or more worth of value around the world at little or no cost, and without permission. That won't change.
As far as services go people should have a right to expect that services will do everything they can to protect assets and the trust given to them for doing that. Of course there are safe ways to buy, sell, and store bitcoins. For example, face to face purchases remain an option, and bitcoinarmory.com is IMO the safest way to cold store bitcoins in existence.