Because of the same physical characteristics that cause glass to be hard while plastic is inherently flexible. Plastic’s molecular adhesion is much lower by design so it’s easy for particles of it to slough off, especially when it comes to things like synthetic fabric being washed in washing machines or billions of tires driving around on roads. When glass gets broken down it breaks into big pieces first and then gradually smaller and smaller pieces until you just end up with sand and can’t be broken down any further by natural processes. Glass shards can be sharp but they aren’t going to make their way into your bloodstream. Eating light bulbs used to be a thing some people did to show off.
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