@lindsaysoderberg,
Because turning off aging feels too cheap. I like having immortal sims, but I want my sims to work for it, to a point.
And managing visits to the cow plant is more interesting, for me, than just farming points to purchase the potions.
A pity Ambrosia has no extra benefits for live sims compared with the potion. Having an actual reason to obtain it would have made it more interesting; as it is, I don't need its age reversing properties due to the method I use, and I don't need it to resurrect sims due to making use of the Book of Life (which doubles as a so efficient way to fill needs that it feels like cheating).