In the case of giant fingers in BG3 and brewing potions in your backpack while walking, I think I simply assumed it to be video game logic and tried not to overthink it. When asked as a DM I completely agree, though. Potions and potion-making were even more restricted in pre-wizards D&D, but the cruel practice of killing giants for buffs goes back to at least 1e AD&D, if not original D&D, with various girdles of giant strength. Those are made of giant muscle fibres. Boosting ability scores, like strength, was very hard in the first place, and these girdles would give you the strength of the respective giant. So, they were supposed to be rare to the point where there may be only one each. Of course, this won't stop players from wanting to create their own.
When my group reached mid-to-high-level, using campaign economy and ecology thus became a necessity to keep the world balanced. It was a bit of a shock to some players that they could permanently break things in the world and do bad things by doing good things. This went as far as the players now enforcing a dragon hunting ban on their allies, after they had once killed too many dragons in an adventure and thereby caused a swamp's troll population to explode. The campaign had begun with villagers complaining about many troll attacks. For strength potions, we use renewable giant hair these days.
