As cat owners, or looking to be cat owners, the question of why cats may not finish their bowl of food is always thought of. Some cats eat half of their bowl, they demand from their owner to get more food so why is that?
First, their whiskers.
Cats have very sensitive and deep nerves in the base of their whiskers, and have nerves at the end of them. Their whiskers allow them to know many important things like if their body will fit through an opening, how far they will need to jump, and if there's anything moving around them. There are two main whiskers near their nose that grows to the size of their body this way they will always know if their body will fit in an opening. Their whiskers are sensitive enough to pick up environmental change such as wind from object movement or temperature change that allows them know what is happening in their environment.
What does this have to do with eating? Cats have to push their nose into deeper bowls to eat all the food, and depending on how sensitive their whiskers are sometimes the pain of pushing their nose that far in isn't worth it for the food. If your cat is never finishing their bowl, maybe consider switching their food bowl out for a shallow dish this way they don't have to push their whiskers back to eat.
Second, their instincts.
Cats have small stomachs, and will typically eat until they are satisfied. This can be a much smaller amount then owners think. If your cat just doesn't finish her bowl at one go, don't worry. They are going based off of their instinct to only eat until they are satisfied, and they like to leave some food for later. This way they can process the food they have had, and then return and eat again if they are hungry.
Third, taste.
We may not think of this, but their dry food can go stale left out during the day. No one likes eating the same food every day, and even worse stale versions of the same food every day. Just likes us, cats don't enjoy this either. Mixing up the type of food you feed your cat could keep them satisfied such as one wet food and one dry food a day.