Here's why:

* Thermal conductivity (k) is a material property. It represents how well a material conducts heat. It's a constant for a given material under specific conditions (like temperature).

* Thickness (d) is a geometric property of the wall. It influences the amount of heat that flows through the wall, but not the inherent ability of the material to conduct heat.

How thickness impacts heat flow:

* Increased thickness = Reduced heat flow: A thicker wall offers more resistance to heat flow. The same amount of heat will take longer to pass through a thicker wall, leading to less heat transfer.

* Thermal resistance (R) is a measure of how well a material resists heat flow. It's directly proportional to thickness (R = d/k). Doubling the thickness doubles the thermal resistance.

In summary:

While the thickness of a wall affects its thermal resistance and heat flow, it doesn't change the thermal conductivity of the material itself.