Maintenance is a time consuming process. A major overhaul on a carrier takes a year or more. IIRC, Nimitz spent close to a year in dry dock at Bremerton undergoing major overhaul. Not all of our assets are available at all times. Refueling the nuke boats is another long term process. IIRC, refueling the Eisenhower was deferred during the Obama administration. That may be happening now.