Why Coax loss comes mainly from conductor resistance and dielectric heating, and both get worse as frequency rises. Skin effect pushes current into an ever thinner outer layer of the conductor, raising effective resistance, while the dielectric absorbs more energy at higher frequencies. That is why a cable rated at a fraction of a dB per 100 feet on HF can lose several dB per 100 feet at UHF, and why short runs or low-loss cable matter most on the higher bands.
Watch out The impedance choices are a trap: characteristic impedance is set by the conductor diameters and the dielectric, so a 50 ohm cable stays essentially 50 ohms across its whole usable frequency range.
Higher frequency, higher loss. Impedance is set by geometry, not by frequency.