Because room temperature (≈298 K) is over the typical rotational temperature but lower than the typical vibrational temperature, only the translational and rotational degrees of freedom contribute, in equal amounts, to the heat capacity ratio. This is why γ ≈ ⁠ 5 / 3 ⁠ for monatomic gases and γ ≈ ⁠ 7 / 5 ⁠ for diatomic gases at room temperature.