Currently, plastic packaging is measured in the tens of millions of tons per year, while coal is measured in the billions of tons.
The plastic in our bodies is not generally getting there from environmental pollution, it's getting there from us using plastic to wrap and cook the majority of our food and drink and cosmetics etc.
Not burning the plastic risks its turning into microplastics, which will tend to interfere with the physiology of all plants and animals.
There's a lot of talk generally of running carbon sequestration technologies and how important that will be. Burying plastic waste is exactly doing that, without spending the extra power to actually extract the carbon from the air.