Just curious . . . Seems like it would less expensive and safer.
Maybe the gas reacts with plastic. Glass is non resistant to acids and corrosion.
Well, Neon is an inert gas. It seldom if ever will react with anything. The reason being is that the tubing can get substantially hot. Plastic melts at relatively cold temperatures...for instance, even hot water can melt plastic. It wouldn't really be safer to have the plastic tubing melt on you...now would it? If the neon gas escapes...it isn't really any big deal. Neon is already part of our natural air anyway..that is where they get it from. What is a big deal in event of failure, is if the gas isn't a pure inert gas. ONLY the red-orange lights are literally Neon lights. The other colors are actually fluorescent lights (with the exception of some Argon/some Krypton/some Xenon lights). Fluorescent lights use a very toxic gas to function...MERCURY VAPOR.
