The two basic reasons is that plastic is made from the byproducts of some hydrocarbons which we use for other reasons, and we have plenty of this kind of byproducts because we use a lot of hydrocarbon-originated products for other purposes. The second reason is that plastic is a very light material, and from small masses of hydrocarbon byproducts we are able to create very large plastic objects. It doesn’t take a lot of the thing to create plastic objects.
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