Except the cells are formed with rigid members which transmit sound (and heat) quite efficiently. Sound isolation can't just be done with mass. There has to be a decoupling you can't get with a brick wall alone.

Even advanced heating/cooling efficiency techniques with wood framed houses involve some form of isolation - no single material will exist all the way through the wall. They're decoupled with several inches of foam. Even modern studs are being built in a way to minimize the ability for them to transfer heat (Tstuds).