plastic degrades I think due to UV and vibration, I had a Thule box that i left on the car for 10 yrs, it would develop new cracks every year so every year I would patch with fibreglass drywall repair tape & epoxy, it became a maintenance thing , finally somebody stole the car/ the rack/ the box
You can probably get the metal fasteners apart with penetrating oil and then use anti sieze once per year