I am going to propose a theory, which may not be valid but I think it is, concerning the relationship between a cat owner and their domestic cat.
This relationship needs to be intimate and close if both parties are to get the best out of it. There needs to be interaction between the two. This interaction needs to be calm and gentle emanating as it will from a loving bond.
The key, therefore, is that the needs to be an interaction and this needs to be sustained for a close relationship to be formed. There are times when a cat owner and their cat must interact. Two of those times are when the owner feeds their cat and when he/she lets their cat go outside when a door needs to be opened.
In order to be fed a cat will sometimes ask their owner and in order to go outside, if there is no cat flap, a cat will ask their owner. The owner has to respond. In doing so the owner will probably talk to their cat and therefore there is an interaction. One also creates a relationship in which the cat needs the human to do something for him/her. It reinforces the dependency of the cat upon the human. I think this is helpful to the relationship because cats, after all, are domesticated wild cats. That domestication needs to be reinforced.
If a cat owner feeds their cat dry cat food it can be left in a dish all day for the cat to graze on. There needs be no interaction between the two. And, of course, if there is a cat flap, the cat can come in and go out at will with no interaction between human and cat. There is no need for the two to interact. Either cat or human has to positively instigate a human/cat interaction under the circumstances.
If a cat owner is busy or disinterested in the relationship then it can be broken or weakened under the circumstances of dry cat food and a cat flap. This can lead to relinquishment.
Obviously cat flaps are very convenient and the same can be said about dry cat food. It is this convenience in fact which can undermine the relationship, I feel. There has to be some input into the relationship. It cannot be too easy which is why I suggest that there maybe an upside in not having a cat flap if you want to let your cat go outside.
It almost goes without saying that to feed a cat dry cat food permanently is unwise and therefore I would feel quite confident that most people feed their cat wet cat food and in doing so it cannot be left out for a long time because it goes off. This means preparing fresh cat food which means in turn an interaction between cat and human.