Why is the Chinese river dolphin thought to be extinct?

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Illustration showing various cetaceans, including baleen whales like the blue whale, fin whale, humpback whale, gray whale, sei whale, northern right whale, and bowhead whale, as well as toothed whales such as the sperm whale, narwhal, beluga, Baird's beaked whale, killer whale, dolphins, and porpoises. Each species is labeled with its scientific name and typical or maximum length, with a size scale in the corner for reference. — cetacean Whales (order Cetacea).

The baiji, or Chinese river dolphin (Lipotes vexillifer), is widely believed to be functionally extinct because its habitat in China’s Yangtze River became increasingly crowded and polluted.1

While the International Union for Conservation of Nature and Natural Resources (IUCN) officially classifies the species as critically endangered, the last confirmed sighting of this cetacean occurred in 2002. Although there were reports of an encounter in 2007, no further evidence has emerged to suggest the species persists in the wild.1