When negative frequencies cannot exist in practice, why do we calculate power in negative frequency?
Negative frequencies do exist. The vector base for Fourier transforms (and therefore power spectral density) is the set of complex exponentials $e^{j\omega t}$ where the frequency $\omega$ can be positive or negative.

A real sinusoid can be decomposed into complex exponentials:

$$\cos(\omega t) = \frac{e^{j\omega t} + e^{-j \omega t}}{2}$$

There you have your practical signal (the cosine wave) expressed in terms of the Fourier vector base (complex exponentials). Both sides of the spectrum must be considered in order to calculate power.

This is similar for any other real signal. In general, for real signals, the power spectrum will be symmetric and you must consider power in both sides of the spectrum.