Cat always hides

I have this cat who's 11 years old that I've had since she was a kitten. She hides all the time. There are days that I don't even see her. My other cats aren't like this. A couple times she sat on the couch or bed with me and drooled. She's never been hurt in any way.

My question: how is she happy living like this? This doesn't seem like a good quality of life, and I don't get any enjoyment either.