Been asked over the years many times, answers vary but agree that the rule is socks must cover the whole shin pad and they come quite high now, also to add protection to the knee.
//And why is Serena Williams wearing fishnet tights when playing some of her matches at Wimbledon?//
Because as a world famous sportswoman she can wear anything she likes but the real unfortunate reason is a health issue as quoted by the above as a quick google search tells us.