Because lacking ethical prohibitions always gives one an advantage. People who lack ethics can always choose to act ethically when it confers a social advantage, or to act otherwise when that's more advantageous. Ethical people are by definition often forced to act at a disadvantage.

The only advantage that individual ethical behavior gives is through supporting the health of the group, thereby supporting members who depend on the group. People who aren't loyal to a group can shop around, play one against the other, etc. Even better, if you automate the interactions between people, there's no need or benefit for ethical behavior. Profit comes from pleasing the algorithm rather than any sort of loyalty.

Edit: The algorithm that capitalism uses is maximizing value against cost. All ethical complaints about it are asking people to accept less value/more cost. Capitalism insists that if everyone maximizes value against cost personally, that sends a control signal to the collective that will maximize the greater good. This is the ethic that we're automating.