There are certain types of cats that are very affectionate - Siamese, for instance, although they can do overkill very quickly. I’ve read that cats only have a memory span of about 4 months so that may have something to do with their lack of affection. They just don’t remember their owner died.
I don't believe this is true. Before we started getting home vet visits, my cat went to the vet once a year, and when she saw the carrier come of of the closet she was long gone.