I know those "tamed, socialized" and adopted kittens. I know them well. Many of my pet sitting cats began life as feral kittens. Today, they live strictly indoors as terrified adults.
They're the cats who run for the nearest hiding place when the doorbell rings. They're the ones who cower under a bed or in the deepest, darkest corner of a closet when a stranger is in the house. That stranger could be the cat sitter who has been visiting them for years, but they still hide.
I've found frightened cats in box springs and behind washers and dryers. I discovered Holly, who was adopted from my rescue and had known me for years, hiding behind the speakers of her family's stereo system. Frosty somehow managed to get inside an armoir, where she was safe from the invader who was there to give her food. Meanwhile, her brother, who's not the smallest cat I've ever known, took refuge underneath a treadmill.
In time, many of these hidey cats become my friends. But others never do. And some find a new hiding place the second I discover the first one.