Dude. Please don't embarrass us with "coat check" examples anymore. Even a parking lot would be more suitable of an example, or even a storage container facility. Those have contracts at least and expect you to store things for extended periods of time.
The point is the difference between using a service in a way that requires the normal level of security and using a service in a way that requires an extraordinary level of security from the provider.
You're saying I can't sue the parking garage of one of their employees breaking into my car and stealing it?
Yes, but don't expect to get back the $5 million if you store a prototype car there.
You're saying that I can't sue the shipping container company for leaving their keys outside of my container and letting someone just rob me?
Yes, but don't expect them to cover the costs if your shipment was diamonds, unless they knew and agreed to extra security appropriate to diamonds.
Give me a break.
In your world, every business would have to provide security adequate to cover the most bizarre uses of their service. FedEx would have to have a team of armed guards follow every truck they dispatch just in case a package had millions of dollars worth of diamonds in it and the owner of the shipment made the shipment details public so thieves knew just what to target. But in fact, that's not how such services operate. They have precisely-defined liability limits and they require shippers to declare high-value operations and pay extra if you want them to insure them.
Yes or no, do you believe FedEx is legally obligated to defend every package they ship in a way that's suitable to protect millions of dollars worth of diamonds from an inside job? If yes, how do you think they should pay for that? If no, how can they be negligent if their security was adequate for ordinary shipments?