One of the parts you’re talking about would be the wing box. From my understanding of it, it’s an electron beam welded titanium structure that was both incredibly difficult to manufacture and also impossible to repair. Every US Tomcat that you can see on display today has had it broken before they went on display. Not only do spares no longer exist and production halted, but the original machinery used to create these parts were also destroyed.

Additionally, the hydraulics were only one part of an incredibly difficult beast to maintain. It’s the largest and heaviest carrier based fighter to be put into commission, and it was made in the 1970s. It’s not like you can hook up a laptop to the plane to diagnose an issue, you’re going to be pulling out panels and inspecting each actuator and hydraulic line while taking multimeters to look over each cable to find issues, not even to mention the fact that it still relied on a mechanical INS to derive positional data for its avionics. It was enough that it was a significant factor in pushing it into retirement.

The commonly quoted figure is 50-60 man hours of maintenance for every hour of flight compared to only 10-15 for the Super Hornet. It wasn’t just a maintenance heavy plane, it was a diva.