The F-18 is an aircraft that is capable of being operated from Carriers. But in addition to that, the F-18 have seen considerable export success to various Air Forces that never had the slightest interest in operating the planes from Carriers. Nor does it seem particularly obvious in what ways the Carrier borne jets are inferior to their land based counterparts. It isn't just the F-18; every navy jet have had at least some foreign operators, and those foreign operators generally didn't operate the planes from carriers.
Given the high R&D costs of aircraft, what would be the downsides of having a SecDef that just says "Hey Airforce, instead of developing two planes, let's just develop one CATOBAR capable plane for the navy and operate that from normal runways too"?
Since the past is easier to discuss than the future, what did the F-16 gain over the F-18, and what did the F-35A gain over the C?