Cats are strange creatures with unusual habits, and for many cats, one of their most unusual habits is to sit in a bathtub that’s empty of water. Most cats dislike water but they like sitting in a tub that’s often filled with water.
Apparently many cats like the cool surface of a tub, especially on warm days. Cats also like the protection a tub offers, much like a cardboard box, that allows the cat to hide out of sight while still being able to jump out when it wants.
So cats like tubs for the same reason they like cardboard boxes. If the world ends tomorrow, perhaps the thing cats will miss will be fresh cardboard boxes but at least they’ll still have tubs.