Because of the rate at which humans are consuming and extracting them. the real question is are you talking about natural resources? if you are talking about natural resources that is the reason. Every natural process has a long geological time period through which it replenishes itself. For example, the resource of water... putting aside that water is unevenly distributed around the world... it is scarce because humans remove vast amounts of water from aquifers so the water table of that areas drops extremely. the perfect example of how scarce water really is would be how South Africa is reaching day zero.
