Also,

We have different everything. Voices (like faces) are noticeable because we use them as a primary means for communication within our group, and our brains have evolved to zone in on the differences nonchalantly because it was a matter of life and death back then. Nothing about two people is exactly the same, things may be very similar. Most of these things go unnoticed, since they didn’t play as significant a role in our evolutionary history.

I imagine a dog asking on doggedit: “why do humans all have different smell?”