Well couple of reasons I can think of, If you operated an F18. it flies itself off so probably the computers expect it to be pretty much level on take off.. The other main one would be landing, If they miss the arrestor cable they do a bolter and go around again, so they're in both cases probably carrying to much speed to use a ramp?
That said the Russians catapult launch of a ski slope. but the cross deck used in landings does not have one.
JUST GUESSING.... ANYONE?