The reason why is because it can often be self-defeating. Self advantage and self-preservation can come into conflict. And I think the term greed is a term that is meant to communicate the concept of self-advantage seeking, that can jeopardize self-preservation. Not a term that is taken to mean rational self-advantage seeking, which should not threaten self-preservation. However the difference is difficult to come to from a purely subjective perspective, and it is easy for an individual to assume they are making decisions and seeking opportunities to grant themselves an advantage, that may, in fact, be limiting the overall potential of prosperity.
For instance greed is what motivates people to dismantle the EPA, however, the potential threat to future generations of your offspring is an example of how greed can come into conflict with basic self-preservation. If the pollution should become so severe that future generations cannot endure as the result of self-advantage seeking greed, then greed has threatened self-preservation.