Question

Why plastic bags should not be disposed off by burning

Answer 1

in short we can say
Burning of plastics releases a lot of toxic fumes along with carbon monoxide, Dioxins and furans. Burning and Land-filling are not a environmentally friendly ways. ... Well, plastic cant be disposed off. There are plastic waste disposal solutions like reused or recycled.
Burning resources , creating toxic pollution, whether for energy generation or waste management disposal, makes no sense if we are serious about reducing greenhouse gas emissions and addressing climate change.

hope it works

Answer 2

Because plastic is man-made polymer material that can be moulded and desired shape or drawn into fibres , it can be destroyed it can be
in many shape but cannot destroyed at all.