Why amphibious assaults usually land away from ports

Why is assaulting a port usually worse than landing on a long open beach?

Ports concentrate defenses and restrict the attacker even though they offer superior unloading facilities, so an open beach is often easier to seize first.

A port lets ordinary merchant ships unload far more efficiently than landing craft on an open beach, which makes it an obvious objective after troops are ashore. That same value ensures ports receive forts, garrisons, artillery, and layered defenses accumulated over generations. Their approaches also restrict the number and direction of landing craft, while established roads and railways help defenders reinforce the position. Falmouth, Dieppe, Valletta, and Calais illustrate why attackers usually choose a less defended beach, secure a perimeter, and take the port from landward afterward.