Spare parts in airplanes is a funny business. It's already hard enough trying to source the replacement part for your mass-produced 2008 Toyota Corrola; getting

Spare parts in airplanes is a funny business. It's already hard enough trying to source the replacement part for your mass-produced 2008 Toyota Corrola; getting a replacement canopy for your F-35 is an ordeal. Usually, a spare part does exist. But it's probably located on an airfield in Australia, a confidential testing plant in Arkansas, an aircraft carrier in Cyprus or a warehouse in Illinois. Finding that part, getting that part to the plane, and then bringing the plane up to flight-ready standards is a hellish process, and it's unsurprising that it's grounding planes even with supply chain databases to help. In wartime, desperate (or thrifty) countries will cannibalize the less-fit models of their newer aircraft to serve as parts for the rest of their fleet. That way you can accelerate deployment by fielding less planes that have a higher ratio of on-site replacement parts. But the US isn't very pressed at the moment, and it makes sense that they're willing to accept a low readiness rating to iron out the kinks without turning an F-35 into a hangar queen.