The charger port is also on the bottom, and if you have the phone plugged in and also feeding audio through an audio cable, you’d presumably want the wires to go the same way.
Usually people put their phones into their pockets upside down, since it will be the right way up when you pull it back out without having to reorient it in your hand. In older, more innocent days when we all listened to music with wired headphones, the port would be facing out of your pocket, so you could keep it wired in without fumbling to flip the phone the other way around when you take it back out to use it.
Couple reasons:
The charger port is also on the bottom, and if you have the phone plugged in and also feeding audio through an audio cable, you’d presumably want the wires to go the same way.
Usually people put their phones into their pockets upside down, since it will be the right way up when you pull it back out without having to reorient it in your hand. In older, more innocent days when we all listened to music with wired headphones, the port would be facing out of your pocket, so you could keep it wired in without fumbling to flip the phone the other way around when you take it back out to use it.