Well I hesitate to even venture into this question, because technical terms are being misused creating the problem.
Resistance, and resistivity, is something that arises from Ohm's Law.
Namely, for a certain class of materials (mostly metallic conductors) , if all other physical parameters are held constant (difficult to do), the ratio of the current flowing to the applied Voltage, is constant.
So Ohm's law simply says: R is CONSTANT .
And R does not vary with frequency either. with varying currents, Ohm's law applies to all instants of time, so with AC Voltages and Currents the two are ALWAYS in phase.
The practical problem arises, in that when you have a current flowing in a resistive medium; say a wire, there is a magnetic field set up, that surrounds the current flow, and that magnetic field is also inside the wire, and the magnitude of the field depends on the ENCLOSED current. So the center of the wire has a lower current, so it generates a smaller magnetic field.
If the current varies, then the magnetic field is restricted in its movement, or change, by the velocity of EM wave propagation (c).
As a consequence of this time lag, the current carrying conductor now exhibits, an Inductance effect, so the equivalent circuit is no longer a simple resistor with constant Ohmic resistance; it is a series circuit of a resistor in series with an inductor; approximately 3 nano-Henries per centimeter of a straight wire.
So you now have an AC impedance that is Z = R + j.2.pi.f.L
So now the current will be less, and as you raise the frequency, the inductive reactance will increase linearly with frequency, so the current will drop.
The resistance has not changed one iota; the impedance has. Eventually, you will end up with the current in the center of the wire going completely backwards, compared to the outer layers. That backwards current further diminishes the current for a given Voltage, so the wire center is now more of a nuisance than a useful conductor. So you might as well get rid of it, and use a hollow tube.
This is the essence of "Skin effect", it has nothing whatsoever to do with the resistance or the resistivity of the conductor, which remains completely frequency independent. It is the AC impedance that IS INCREASING, not THE RESISTIVITY.
If it is frequency dependent, it is NOT a RESISTOR, complying with Ohm's Law; it is a complex AC circuit involving Inductance, and also Capacitance, when you get into it.
Words, have meaning, and when scientists use the wrong words; specially ones that also have colloquial common meanings; it creates havoc for all; this question for example.