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My guess? It's actually the consistent use of the cartilage that keeps it healthy. Just like how weight bearing exercises can promote deposition of calcium in bone, I'd imagine there's a similar mechanism for cartilage?

Hip joints (especially in modern era) get far less weight bearing on them - especially as a force / area, since we're sitting all the time.

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Motion and loading is definitely integral to cartilage preservation. Unlike most tissues in the body, articular cartilage does not receive oxygen and nutrients from blood vessels, but rather directly from the oil-like synovial fluid that helps lubricate the joint surfaces. I imagine the thin squishy layer of cartilage behaves like a sponge, deforming compressive forces drive synovial fluid out and removing the load (taking weight off the joint) sucks synovial fluid back in, sort of like inhaling / exhaling.

Here is a pretty good basic science review article about articular cartilage:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3445147/

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