Because they are less in weight, lonlasting,durable,cheap,and it can be recycled,and plastic wont react with air and water,they are not corred easily. thats is why  plastic containers are favoured for storing food.Hence plastic is very useful than metals now a days ,in the kitchen and  more use of plastic leads global warming too.
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we know that plastic is a non-corroding and non-reactive material.hence,the plastic containers will not damage the food i.e.they donot make a change in its quality or flavour.thus,the plastics are used to store food materials