While the answers by topo morto and user45266 are both correct, they do not really go into the "why" from the physiological side. It looks like that the perception of different frequencies is no different from sensory perception in general, which is logarithmic in nature. This empirical fact is known as the Weber-Fechner law.
Same goes for example for visual brightness or the loudness of sound. When we feel a certain difference in brightness our loudness, it corresponds to a certain ratio (as opposed to difference) of the input stimulus (that is the physical intensity in this case).