I was hoping to can some food to take backpacking and looking for metal canning jars for home canning, but as far as I can see, they don't exist. Is there any reason why canning jars are made of glass? Metal seems like it would be lighter and sturdier.
Metal may be lighter and sturdier, but glass is nicely non-reactive and easily cleaned for re-use. It's also easier to see what it looks like inside without breaking the seal, to see if there is something that doesn't look right. It's a lot easier to do safely in home environments.
I'm not sure if the difference in the thermal properties of the metal ring vs the glass lip are important in water bath canning (e.g., the ring expanding more than the glass).