Tennis balls die slowly, and almost entirely out of sight.
Roughly 325 million tennis balls are produced every year. Most live a few hours of play and then sit in a landfill for 400+ years. They're rubber, nylon, and felt: recyclable in theory, ignored in practice.
Country clubs, academies, schools, casual courts. The waste is everywhere, and it's polite. Nobody is mad about it. That's exactly why nothing changes.