Why an escort carrier is not automatically an LPH

Could escort carriers simply serve as the basis for helicopter assault ships?

Their hulls could, but they need substantial adaptation to embark and sustain a landing force in addition to ship and air-group personnel.

An aircraft carrier supports the ship's crew and air-group personnel. A helicopter assault ship must also accommodate and sustain a landing force, with its own equipment and stores. Weapons need secure storage, and the troops need facilities for the time spent at sea. Escort-carrier hulls could provide a useful starting point, but they would require conversion for those additional demands. The postwar example likewise involves adapting carriers to support landing forces.