glass is a lot heavier and way more expensive (glass is made out of silicon).
There are a lot of different types of plastics the common ones use in packaging are LE-PE (light density polyethylene) and PP (polypropylene). They are both thermoplastic, they melt then heated. Silicone is also a form of plastic, it's thermoset (it chars effectively and it doesn't melt) - and it's awfully expensive. There are other plastic, e.g. nylon (PA6) that are still expensive but much cheaper than silicone.
