Because they can. Folks love to have "ocean front property". Of course, the more people that desire this, the pricier it becomes to obtain. (its more prestigious to own ocean front property)
Most barrier islands on the US east coast started out as spits and sandbars - temporary deposits of sand created and destroyed by tides and storms. Some spits and sandbars last only a brief time while others can last for years until a large enough storm or tide arrives to shift the sand elsewhere. Over the years, people reinforced the "islands" off the east coast turning them into placed to live by adding tons and tons of rock and soil, growing them into what they are today. But, just like a sandbar in a river, they are only temporary. So more rock must be added annually in order to keep the "island" from disappearing back into the sea.
People have foolishly tried to control nature for years. They build houses on the side of cliffs, in riverbeds and on the sands of the sea. Shrewd contractors build these houses when all is calm, but when the wind and rains come, the houses wash away leaving the foolish owners perplexed by it. (Why did the government allow this to happen?) Then they foolishly rebuild in the same place. Sure, people may try putting up barrier walls, levees, dams, and such, but nature almost always wins in the end.
Shuhak 