They are, but your protections are pretty minimal. We have more theft by our government (civil asset forfeiture) than by actual thieves. Not just that, banks themselves have proven to be unreliable with boxes independent of government theft.

Example CAF case: https://www.google.com/amp/s/www.latimes.com/california/stor...

Example bank fuck-up: https://www.nytimes.com/2019/07/19/business/safe-deposit-box...