A very good question, one to ask NZTA possibly? I think it is because the "expiry" date is like a "best before" date used on food, which says something along the lines of "it really would be a good idea if you replaced these parts after this time" rather than "you must replace", which combined with the practicalities of checking the dates on the bags in a car kind of put it outside the realms of most inspection processes. I have heard anecdotally of countries that use the airbags as a barrier to used imports, where they do insist on them being changed which makes things cost prohibitive, whether true or not I cannot say 100% as I have never tried to import an old car into those places.
I also understand that with most airbag inflation reactions, the material gets less volatile over time, so what happens as the airbag gets old is that it will still work, but can possibly deploy a little slower / not as large as originally designed. Which whilst not ideal, is not the end of the world.
The problem with the Takata style airbags is that they are having the opposite effect, the presence of water, absorbed from moisture, in the chemical acts as an accelerant to the reaction so they actually deploy quicker, ie. more violently, and as a result of that uncontrolled deployment the metal casing is not failing in the way intended which has resulted in pieces of metal shrapnel being produced and sent into the cabin of the car. Which is unfortunately much more of a danger.
Latest situation with the Takata company would appear to be that Key Safety Systems are looking to take over the "embattled" Takata for US$1.59 billion. The approximate number of cars affected is now over 100 million.