Why doesn't a microwave heat the air around food, in addition to the food itself? I'm reading Harold McGee's &quot;On Food And Cooking&quot;, and in the &quot;Microwave Cooking&quot; section he writes:

Since the air in the oven is not heated, microwave ovens can’t brown meat surfaces unless they’re assisted by special packaging or a broiling element. (An exception to this rule is cured meats like bacon, which get so dry when cooked that they can brown.)

My question is: if the radio waves generated by a microwave are passing through air in order to reach the food that is cooked, and since air is made of molecules just like food is, why don't the radio waves affect the air molecules the way they do the food molecules?
With all electromagnetic radiation, including visible light and microwaves, absorption depends on the molecules doing the absorption.  Air is mainly oxygen and nitrogen, and these don't absorb very well at the 2.4GHz frequency used in microwaves, while foods do.  A lot of this is down to the very efficient absorption by water, and almost all foodstuffs contain a fair amount of water.
Note that the air does warm up with heat from the food.  For short cooking times this isn't a big effect, but for things that cook longer it's significant