Because of the way the numerous laws are currently drafted, natural justice is not being served.
Without actual proof of an alleged offence having been committed by an individual, it is probably a breach of human rights legislation to penalise said individual.
Parking rules & regulations are designed to confuse. Often PCNs are issued on a whim hoping to raise money when the alleged infringement is dubious at best. Redressing the balance would encourage parking authorities to ensure their signage is fair & reasonable, only issue tickets when a clear breach has occurred & make the practice of ticket issuing quotas non viable.
They are simply taxation in disguise. (As proof of this most insurance companies tend to discount 1 or 2 minor motoring penalties.)