A ski jump would cost America-class helicopter spots

Why does the United States Navy not fit ski jumps to America-class ships?

Their primary role is amphibious helicopter lift, and a ramp would remove roughly two helicopter operating spots from the flight deck.

America-class ships are large-deck amphibious assault ships rather than dedicated aircraft carriers. Their flight decks must launch and recover as many helicopters as possible to support amphibious lift. A ski jump occupies the forward deck area and reduces the number of simultaneous helicopter spots. The United States therefore preserves helicopter throughput instead of optimizing the ship around short-takeoff fixed-wing aircraft.