Carriers provide a capability that is needed infrequently, and one that can realistically and legitimately be provided by Allies, whereas the need for land based AD is 'constant' and therefore must be a national capability.
In the event of trouble, the first thing a president asks is "Where are the carriers?", not "Where are the F15s or B2s?". You mention that air defence is a constant whilst choosing to forget that a carrier provides that air defence constant - where and when we need it as well as deep strike. That's a real joint asset and great value for money.
Don't the US have as many F15c based in the UK as we do F3s?