Why a coastline cannot be defended at every possible landing point

How can an army muster enough troops to defend every possible beach landing?

It cannot; coast defence has to prioritize likely sites and retain responsive forces because an attacker may adapt even a marginal beach for use.

No defender can place enough troops at every beach while maintaining effective command and control. Amphibious forces can also spend resources to make an apparently marginal landing site usable, so terrain assessments do not remove all uncertainty. Coastal defence therefore depends on selected strongpoints, mobile reserves, and the ability to respond after the attacker reveals the landing.