Knees do not necessarily wear out evenly, sometimes one part of the knee is perfectly fine while another part is completely destroyed. If only part of the knee joint is worn out, why replace all of it?
When Serena Williams was penalized during the US Open, she turned her anger against her accuser—the umpire—rather than on her opponent. She lost. At the same event, when Roger Federer suffered from the high heat and humidity he couldn’t wait to get out of the poorly designed Arthur Ashe tennis stadium. He lost.