Knees are about like ankles. Noticed them more as a kid 'cause they were always getting beat up. Kicked in soccer, twisted in basketball. Thighs, on the other hand, you see a lot more. Right there in your lap as you look down.
Laps are a little more of an everyday functional apparatus.
You can set things on them, a book, your food. They are always good for the family cat too. Cats always seem to want that reciprocal recognition while you are intensely occupied by something else. Then there are the ever popular genitals. Not nearly as universal as feet and legs.