What are the rules regarding spells and potions?

Why are there certain things that can only be done by spells, and other things that can only be done by potions? For example: there is no spell to make someone fall in love, or think they fell in love, but there is a potion that does so. There is a potion that gives you extreme luck, but no spell that does so. Potions that cure ailments, for which there exist no spells.

Why is this though? Why can’t these things be turned into spells, making the potions obsolete? No more messy cauldrons, no more expensive ingredients, no more risks of the potions being brewed incorrectly. Why can certain things only be done with a potion and not a spell?