Why DC resistance is used in MOSFET?
A MOSFET can be either used as a switch or as a voltage controlled current source.

When used as a switch the gate-source voltage is chosen high enough to operate the transistor in the linear (triode) region. In this region it can be modeled as a simple ohmic resistor. Using this approximation DC and AC resistances are equal.

Using the transistor as a voltage controlled current source is achieved by operating it in the saturation region. As a first approximation only the voltage controlled current source is modeled and the drain current is constant and only dependent on the gate-source voltage. Neither the definition of an AC or a DC resistance would make sense. Including channel-length modulation (and similar effects) requires the addition of a resistor in parallel to the current source. This model is only valid for saturation and can be used to calculate the incremental change of the drain current for an incremental change of the drain-source voltage (AC). The resistance has no meaning for a DC, since the model is limited to the saturation region.