It's because the PMC struggle for relative social status leaves no room for morality, or even self-awareness. One's income and access to future opportunities (and ability to avoid falling into the working class) depends completely on controlling a share of a fixed-size pool of social capital, so social competition is ruthless, and there is no place for empathy or regrets.

"Terrans don't have souls. We don't believe in them."  — Benjamin Sisko (mirror universe)