In my thought experiment, since air exerts $14.7$ psi at sea level, the air within the cube of steel should exert the same pressure outward, $14.7$ psi. Right?
The whole thing about balanced air pressure is quite counterintuitive to me. For example why aren't things being deformed by the fact that there are two 14.7 psi forces ramming it in from the inside and outside? Aren't we left to conclude that every single container in the world can withstand a pressure of >100000 Pa?! And if you take an arbitrarily small amount of air (let's say a few thousand atoms) and pack it into a container and ship it to space, will that extremely tiny amount of air still manage to exert such a great pressure?
EDIT: Great answer, @Bill N. However I don't really get why once two equal in magnitude forces are directed at each other, nothing happens. There will still be compression. I mean, when you compress gaseous oxygen at 96 GPa (perhaps against a diamond anvil) it turns to metallic oxygen. So there is still the compression problem.