We don't have to; like most mammals, humans habitually seek out fresh water sources on land, and these are pretty abundant on much of the planet.
Even many aquatic mammals don't drink seawater. Seals get water from their food; ocean fish is much less salty than the water they live in. Manatees move between salty, brackish, and fresh water; and they drink when they're in the fresh water. Polar bears eat snow.
Whales, dolphins, and sea otters can drink seawater. But they're pretty weird.