Quote: a shopper would need to reuse the same cotton tote for 29 years before it caused less fresh water aquatic ecotoxicity than all of the plastic bags that it would replace over this period. Given the improbability that the same cotton tote would be used over such lengths of time, in most cases plastic bags will have less environmental impact.
Why is this? Because the environmental impacts of supermarket bags are dominated by the energy and raw materials needed to manufacture them. Plastic bags are inexpensive because relatively small amounts of energy and raw materials are needed to make them. These same attributes that make plastic bags affordable and light also make them easier on the environment than alternatives like paper bags and reusable cotton totes.