Why do electronic gadgets that is stocked piled for years tend to have defect?

Because the enemy of all electronics devices is dust and heat. Thats why it is impertinent that proper storage and handling is done for every product.

because of DUST, Static Charge due to atmosphere.

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Condition of storage, dust , vibration, heat , humidity and so on

Also maybe the lonelyness of the products , they feel alone and depressed after finally defective ( this part is a joke)