Why has Islam forbidden hoarding wealth?

Islam did not absolutely prohibit the hoarding of wealth; rather, it prohibited the hoarding of wealth from which zakat has not been paid. The severe warning came for the owner of hoarded wealth if he does not pay its zakat, in fulfillment of the words of the Almighty: (And those who hoard gold and silver and spend it not in the way of Allah – give them tidings of a painful punishment) At-Tawbah/34.

The Prophet, peace be upon him, and Ibn Umar clarified that the blameworthy hoarding is wealth from which zakat is not paid. So, whatever zakat has been paid on is not hoarding, even if it is buried, and whatever zakat has not been paid on is hoarding with which its owner will be branded, even if it is apparent.