Looking at this article here by Techopedia, The writer notes that the higher the frequency of a signal in copper cabling, the higher the attenuation across the cable. Why is this the case?
One of the main reasons is skin effect:
Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor, and decreases with greater depths in the conductor. The electric current flows mainly at the "skin" of the conductor, between the outer surface and a level called the skin depth. The skin effect causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor.
Litz wire was designed to minimize the effects.
Twisted-pair copper wire systems attenuate electrical signals due to factors including the interaction of the signal with the copper in the conductors as the described by the level of resistance or impedance in the wire, and the tendency of the signal to radiate, or spread out, from the wire.
Attenuation is sensitive to carrier frequency. Higher-frequency signals generally attenuate more than lower-frequency signals. The phenomenon is not unique to copper cable, but also generally holds true in fiber optic systems, as well, although the measurement is in wavelengths, rather than frequencies, i.e., longer wavelength signals (lower frequency) signals attenuate less than shorter wavelength (higher frequency) signals.
Attenuation refers to a reduction in signal strength commonly occurring while transmitting analog or digital signals over long distances. When using copper conductors, the higher the frequency signal, the more attenuation is caused along a cable length. All electrical signals transmitted down electrical conductors cause an electromagnetic field around the transmission. This field causes energy loss down the cable and gets worse depending upon the frequency and length of the cable.