I imagine you're asking the question "why are reed switches still made of glass?"
Glass is traditionally used. Reed switches find rarer use every day since there are Hall-effect switches and other devices that are smaller, cheaper, and more reliable. Reed switches were invented in 1936, according to Wikipedia.
Nobody retools a buggy whip factory, just as nobody will retool a reed switch factory. If all reed switch manufacturing disappeared tomorrow, it's unlikely a new site would appear to replace the lost capacity.
All of the other mentioned reasons (imperviousness to gas, rigidity, and so forth) can be addressed using plastics. Microchips are currently manufactured in plastics when at one time they came in steel packinging (TO-packaged LM711 comes to mind) as well as ceramic (old intel 4004's, even up to the 8086), but the usefulness of these items and the expense of that technology drove retooling to plastics.