Because greed is the 'dark side' of the inherent drive to take care of our own material needs. The latter is healthy, and forms part of our ethical foundation--in short, it's needed.
Greed is the character trait that results when this drive is allowed to grow out of balance with other moral and psychological imperatives. Until every human develops in a healthy manner, in a healthy environment, greed will exist in at least some hearts.
Today we are still far from such an ideal; indeed, greed is quite often idolized, either explicitly (as in an ideology) or implicitly (as in a cultural byproduct such as 'consumerism.') Small wonder there's lots of it around.