Why are there metal links between the discs of an overhead line insulators?
The insulator string is under tension. 

Ceramics are not particularly good at tension strength, particularly when outside and moving around. 

The vertical part of the stack is alternating metal tension links, where metal works well, and ceramic insulating compressive sections. 


As to capacitance: although I haven’t done a specific calculation, I don’t believe there’s a significant source of loss there. The reactance of large transmission lines is carefully controlled, and the insulators are part of that engineering.