I was lead to believe that the main reason the LHA-LHD's don't have a ski-jump was because of politics.
The NAVY basically cannot tolerate any vessel that can be seen as a direct competitor to it's big Super-Carriers.
A 40,000T+ LHD(A) with a ski-jump and a substantial VTOL/STOVL-jet fleet is a direct competitor with the large carriers and since the number of those is congressionally limited (these days 11 IIRC), the NAVY won't allow for them to exist, the LHA America class was already deemed too much of a carrier.
Fitting an
extended ski-jump ala JUAN CARLOS class would be avery straightforward retrofit and by no means a limit to available deck-area, it would add enough capacity for them to be considered real aircraft carriers so we won't ever see it happening if not the military need demands it.
