Why does Jingdong Express require that the price can only be insured according to the original price of the goods?

Because Jingdong logistics service rules are very clear:

When delivering the goods, the customer chooses the insured price and pays the insured fee according to the declared value of the goods. If the goods are damaged or lost due to the responsibility of Jingdong Logistics, compensation shall be made according to the declared value and loss ratio, and the maximum amount shall not exceed the actual loss amount of the goods. If the insured fails to fill in the declared value or pay the insured fee, it shall be compensated according to the uninsured price.