Why amphibious assault ships combine flight decks and landing craft

Why does the US keep building flat-top amphibious assault ships instead of relying only on LPDs?

The flat top can deliver troops by air and heavy equipment by water, giving a single ship both maneuver and lift options.

A limited amphibious operation needs aircraft as well as a way to land heavy equipment. Helicopters can move light forces ashore quickly, while landing craft carry vehicles and loads that cannot go by helicopter. A flat-top amphibious ship combines those methods in one deployable vessel. That combination explains its appeal to the United States, Italy, and other navies undertaking limited landing operations.