Convenience.
At a convenience store or a fast-food restaurant, It's so much easier to get a drink in a plastic or polystyrene (styrofoam) cup, then throw the cup away when you're finished. It's also so much easier at an event, such as a picnic, to use styrofoam plates and throw them away. The same is true for plastic bags.
We are, in some ways, becoming a nation of "If I want to use styrofoam and throw it away, that's my right. I'm just one person, it won't make a difference anyway."
We are paying for that convenience in the increased number of landfills and the increased amount of garbage (including these convenient plastic and styrofoam drink cups) thrown beside the road.
Not all plastic and styrofoam can be replaced with "better" materials, but consumers can help in little ways, such as replacing plastic bags with reusable shopping bags and bringing your own cup to food places like Starbuck's. At luncheons, picnics, and other food events, I bring my own Tupperware "cafeteria-style" plate and silverware.
My husband and I have made a difference in our town by starting a dropoff Recycling Center, and plastic is our biggest recycled item. We run the Center for free, and our city gets any profits. We probably recycle at least 1 ton of plastic every week.
We are not inheriting the earth from our parents and grandparents. We are borrowing it from our children and grandchildren.