Why do Laboratories take a minimum of 5 sample increments for Aggregate samples?

The reason is standard driven (AS 1141) primarily but it does ensure that samples obtained by the laboratory are representative to the larger body of material on site. Care still needs to be taken in the sampling process though as an ill-considered sampling plan may still miss represent the material on site, on matter how many sample increments you take. So five sample increments is the minimum.