Those potions all last a least a minute, and some like giant strength an hour or more! With a little information-gathering or scouting, the party can easily drink potions before they get into a fight- potions don't make loud noises like spellcasting either, so the folk on the other side of the door won't be alerted.
Like many things, it's probably a case of different table traditions. I imagine some tables just have continuous encounters "happening" to them, so they never have time to gather information, they just react to whatever the GM cooks up. "OK you're in the dungeon. You go to the first room, there are orcs there, roll for initiative." "OK the orcs are dead, you looted them and move on to the next room, there's a weird opera battle going on there between a beholder and a gibbering mouther, they get annoyed that you interrupted so you either join in or roll initiative."
Either that, or players just don't know that they could/would/should information-gather beforehand so they can drink potions before they get into a fight... or, I guess, they're not being pushed/challenged enough so they never feel the need to improve their methods outside of combat.
Actually players apparently not drinking potions before/after combat makes me wonder.. what about spellcasters? Is no one casting the buff spells before a fight either? Some spells last 10 minutes, or an hour, but even a minute spell can potentially be cast before a fight.