• Live music is often slightly out of tune. If the piano at a small club where you are playing a one night gig is slightly low your band is probably going to tune to that piano. I've read of vocalists with perfect pitch saying that this drove them nuts.
• Perfect pitch often drifts as you age. I've read of people who had to stop listening to music when they got into their 50s or 60s because their perfect pitch was now off and everything sounded wrong to them.
* "Concert pitch" of A=440Hz is not universal.
* 12 tone equal temperament is not universal.
* Analog playback systems often run at slightly incorrect speeds.
* Some instruments are difficult or impossible to tune, so the tuning will vary with temperature. Nobody is going to tune a pipe organ in a drafty church every time the weather changes.
* DJ mixes rely on playing tracks at different speeds to allow for smooth transitions between tracks of different tempo[0], which changes the tuning.
And with digital players now, you can speed it up but digitally alter the sound back to it's original pitch.