Beaches are temporary features. Ultimately, a beach erodes because the supply of sand to the beach can not keep up with the loss of sand to the sea.
Most sand is transported from inland via rivers and streams.
The damming of most waterways in the US has thus prevented a major supply of sand from getting to our beaches. Sand can also be transported from beach to beach along a shoreline but this is mostly just a redistribution of sand that is already on the coast. On an uninhabited shoreline, new beach can be created further inland. But, when the encroaching sea comes against people's property, the tendency is for people to try and stop the encroaching sea. They armor the shoreline with seawalls, revetments, jetties, etc.