Sharks are pelagic, which means that they live in the open ocean, they don't need to be near either any coast, the surface or the bottom. Pelagic fish can wander, with some restrictions.

Species that need a coast are in less luck, for example. The next appropriate coast may be far away. The Great Barrier Reef can't just migrate to Antarctica, even if there is a suitable coastline down there, because it can't get there from where it is. There is no continuously habitable area from where it is now to the potential new habitat.