Every year, billions of vehicles worldwide shed an estimated 6 million tons of tire fragments. These tiny flakes of plastic, generated by the wear and tear of normal driving, eventually accumulate in the soil, in rivers and lakes, and even in our food. Researchers in South China recently found tire-derived chemicals in most human urine samples.
You have answered your own question. They are built to be tougher, by using more material.
They still shed a proportional amount of material for the vehicle weight.
Not only that but if they get damaged due to trauma prior to end-of-life (such as a puncture of pothole damage) that is a lot more material that is going to landfill (or tyre “recycling” in countries that have it).