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.
More sand is carried off shore, promoting beach loss.
Additionally, jetties placed perpendicular to the beach, disrupting along-beach currents and causing sand loss downstream of the jetty.