Because that is the actual action of the soundwave/instrument itself. For example, when you hit the skin of a drum, the stick will push the skin down a certain distance. When the skin rebounds, it will come back out, but not quite as far as it was pushed down. The first half of that cycle, therefore will create more sound pressure than the second half. The diaphragm of the microphone will react in kind, and a larger voltage will be created at the peak of the upswing of the cycle than the peak of the second half downswing. This is natural, and is not limited to just drums, though that is often where it is the most apparent.
Next time you look at a kick drum or snare track in your editor, you should notice that the waveform is not symmetrical, but rather kind of "unbalanced" to either the top or the bottom. As long as the volume line returns to the center of the graph when all is said and done, that's fine. But if the waveform at silence "starts" and returns somewhere off the center line, that's artificial offset, which should be taken out.
G.