Why don’t cats ever eat all of their food? Well, firstly, it’s not true that cats never eat all of their food in the bowl.
However, often they do leave some food. The two obvious reasons are probably the correct reasons. Firstly domestic cats don’t have an ‘education’ or training on the need to finish up what is on the plate so they eat what they want to eat and stop. If there is too much in the bowl some food is left behind.
This implies that the normal amount of food placed in the bowl is too much for a cat in one sitting. It is said that domestic cats should eat small amounts frequently. This would work in terms of eating everything in the bowl. So the second reason is that there is too much in the bowl!
I often use wet food sachets. There are two kinds where I live; small and larger sachets. My cat usually leaves very little when I feed him with a small sachet of wet food. He leaves more when I use a larger sachet. The conclusion is obvious: the large sachet it too large.
It is said that humans eat portions which are too large. There is normally too much on the plate which is one reason why there is an obesity epidemic in the West. Perhaps pet food manufacturers are creating portions which are too large for cats (as are prepared meal manufacturers for people). If this is true it may be deliberate so that they can sell more food and make more money.
Someone might speculate that cats deliberately leave some food behind as an insurance policy so they can return later and finish it. I am not sure about this idea. I don’t believe it. My distinct impression is that cats just don’t think like this. Leaving food in the bowl is more a careless approach to food consumption than planning for the future.
Another speculative reason is that cats don’t like to eat the last bits in the bowl because their whiskers brush against the bowl which is uncomfortable. Once again I don’t believe it because I don’t see signs of this.
P.S. This question can only really apply to wet food as dry is left in the bowl a lot because it is overfilled because it does not go off. I am not saying everyone overfills food bowls with dry food but in general they probably do.