Potions, traditionally speaking, were not simple a spell in a bottle - though many of them did have basically that effect.
The not being able to put any appropriately-targeting spell into a potion style has likely been adopted to a) be the way potions have worked in the majority of versions of D&D and D&D-like games, and b) to maintain a higher level of predictability as to what spell effects a non-spell-casting character can manifest (which helps keep casters "special" in the face of having how many spells they can cast in a day, and the raw potency of those spells in general, trimmed back significantly).
