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The Zettai Ryouiki fetish is what it's called, and it's because it accentuates the thighs.
They are not knee socks. Those are over the knee socks.
They are hot for the same reason stockings are hot, though they do seem a little 'younger' where stockings is more a mature woman look. But I wouldn't say this is a 'schoolgirl' thing.
They visually slim the lower leg relative to the thigh, which is hot. They also cover up the feet and legs while leaving the ass and genitals exposed. You can have sex with them on.