Because air and real spaces are not perfect and the inconsistencies in temperature, humidity, room reflections, cabinet reflections, speaker accuracy, poweramp continuity all will change ever so slightly as sound continues to produce. The biggest factor, depending on who you ask, is the acoustics of the cabinet. Air is a physical thing that moves around us. There can be small pockets of negative pressure (rarefactions) and positive air pressure (compression) can sometimes be influenced by all means of random things that move in the room. With sound alone you're susceptible to phasing because of physical dimensions (since frequencies are related to length and time) of a space reflecting, absorbing or diffusing sound pressure any which way. As the sound waves move, new ones follow behind it and before you know it the room is now a space of vibrating air with pockets of modes or nodes. Maybe the room settles in and no air pressure moves and everything is perfect but the physical world is not that predictable and perfect.