Because piacere means to please. You have to say who or what is being pleased. "Piacciono le mele" therefore could mean the apples please, but that doesnt really make sense. "A loro" or "to them", is needed making the sentence literally "to them the apples are pleasing". To my knowledge there isnt a way to directly translate "they like apples" into italian.
It would mean something else.
"Ci piacciono le mele" = "A noi piacciono le mele" = "We like apples"
"A loro piacciono le mele" = "Gli piacciono le mele" = "They like apples"
(Please note that many people don't like to use "gli" for "a loro" in written Italian, but it is widely used in spoken Italian.)
http://www.italian-verbs.com/verbi-italiani/coniugazione.php?id=6727 Piacono with one 'c' is also correct.