Its only because the plastic takes ages to decompose. The ammount of plastic that I chuck away at work is shocking, we could probably make a small island out of it... Plus, the fact that oil is on its way out isnt really helping, and the fumes given off during the manufacturing of the plastic. We have resorted at work to buying recycled plastic to make all of our products out of now, mainly vending cups and CD/ tape cases.
Buy one of the cloth type bags for the £1 or whatever it is, last you a lifetime.