Knees are not foam. They are living organs. The strain is taken up by ligaments and muscles, not (just) cartilage. If you locked your knees out, you would have

Knees are not foam. They are living organs. The strain is taken up by ligaments and muscles, not (just) cartilage. If you locked your knees out, you would have to take the strain in your cartilage. That would hurt a lot. More weight does put more strain on those ligaments, and that can result in an injury. And you do wear your shoes out more quickly, which causes you to land unevenly and puts lateral strain on ligaments not designed to take it. The results are repetitive stress injuries that can take some time to heal. But if you're practicing good form, increase mileage gradually, and (ideally) do some strength training, overweight runners can run forever.