If your cat has grown up with humans as "an only child," it may be that she simply accepts humans as her appropriate companions and has no interest in forming bonds with other animals, especially if you are trying to introduce the other animal on her home turf or personal territory. There may also have been some sort of trauma in her past that causes her to be leery of other animals. If you ever wish to introduce another animal into the home as a full-time resident, be sure to get advice on how best to do that. Most cats, even the most seemingly solitary ones, will adjust to another animal companion when given enough time and space.