Why Feed line loss comes mostly from conductor resistance and dielectric absorption. Air-insulated hardline uses a large center conductor, a solid copper or aluminum outer shield, and air (with only thin spacers) as the dielectric, so both loss mechanisms are minimized. That is why it is used for long runs, VHF/UHF, and repeater installations, at the cost of being stiff, heavy, and expensive.
Watch out Flexible coax at 50 or 75 ohms uses a solid or foam plastic dielectric that absorbs RF energy, so it loses more per 100 feet; impedance itself (50 vs 75 ohms) is not what determines loss.
Air beats plastic: the more air in the dielectric, the lower the loss. Hardline wins.