It could make perfect sense for magic consumables (potions, scrolls, etc) to lose their magic properties. Even if you can completely perfectly seal up a potion physically, that doesn't necessarily contain its magic - which could leak out/"evaporate"/otherwise fade over time. Making consumables that stay magical forever might be too much work/cost of the alchemist/enchanter.
Whereas permanent magic items like weapons and armor are crafted in such a way that that their magic is perfectly kept in and/or renewed with each dawn. No one is going to buy a +1 sword that fades after 2 weeks (although sleazy enchanters & merchant might try to sell such swords as actually being permanent...)
I'd probably be more in favor of some other method of restricting long rests/making time meaningful, but I'd accept this method too.
