What is the purpose of the bags on airplane drop down oxygen masks if they don't inflate?

Because they’re cheap?

I think they must be a resivior of some sort, either for the O2 coming in or the CO2 going out. Or possibly a visual indicator for the wearer or others to confirm if one is actually breathing, passed out or possibly expired, for it can be quite loud and windy in the cabin If rapid decompression were to take place…
Just a guess.

they connect to an oxygen tank. The reason it may not inflate is because there’s not enough pressure to inflate the bag since it is disappating into the cabin. There’s still o2 flowing through it, though.

The bag is there to send o2 directly into the lungs of the passengers, instead of through the crack that is sucking all the air out.

Having the bag also requires passengers to remain in their seats, reducing the mass panic that might otherwise ensue.

There tethered so you remain in your seat, preferable belted so there’s a chance for more accurate body count later.

The bag is connected between the oxygen line and the mask which the passenger places on his face.

So… why not connect the hose directly to the mask, right? What’s the bag doing there in the middle not inflating?

I dunno… interesting question.

It is what is called a “rebreather” The system only sends o2 when you inhale. when you exhale the air you exhale goes into the bag and when you inhale again the o2 and air you have exhaled mix and keeps you from getting an excessive amount of o2.