Aluminium does not react very well with the oxygen in the air but it is in fact quite a reactive metal. The reason it does not react is that is has already reacted, but the aluminium oxide has formed in a thin, tightly bonded layer that protects the aluminium foil from further attack. This aluminium oxide layer is so thin that is it not noticeable.
It is possible to remove this layer with mercury II chloride and watch the aluminium reacting rapidly with the oxygen in the air.
