Why are home canning jars made of glass and not metal?
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).