Seeking out every person with a warrant would take more staff-time and resources than many areas can afford. As such, only the most serious warrants are actively sought out.
Others are either passively tracked (officers have a description or image, so that during normal duties, they would recognise such persons) or only heeded if a check of a person already detained shows one (of the warrant is very minor).
As well as this, many of the people with "mid-range" warrants will make efforts to avoid police, such as by moving frequently; not reporting address details; staying with various friends or family, at their homes or businesses; being mobile as much as possible in public areas where few law enforcement personnel are likely to be present, let alone likely to recognise them as subject to outstanding warrants.