Q: Why are the kick drum and the bass always panned to the center of mixes?
A: In the “old” days of vinyl records, the low-frequency instruments were panned to the center of the stereo field to physically optimize how the stylus tracked in the groove. Still, it is possible to find early stereo recordings where the drums and bass are panned to one side or the other, such as some early Beatles stereo mixes.
Today, from a technical standpoint, it is not as critical to center the bass and kick drum in the mix. Digital playback does not face the same constraints that vinyl playback did. However, most listeners are accustomed to the low-frequency instruments being centered. And, this practice does provide a solid base (no pun intended) for stabilizing the stereo image.
Having said that, if your muse tells you to pan the kick or bass, try it! Maybe it will be just the thing for your mix. Just be aware that it may sound odd to some listeners to have the lows off to one side, especially in ear buds or on a car speaker system.