Most modern explanations posit that people are mere biological machines–complex ones, for sure, but mere machines nonetheless. ALL behavior is to be explained by Platonic Material and Efficient causes, and not at all by Final causes.
You “love” your spouse? It is mere cause-and-effect, how machines work. The subjective feeling of “love” is an illusion, a mere ignis fatuus.
Unless you actually chose, the action has no more (actually less) life meaning than a hammer striking a nail. In other words, no real morality and meaning as we generally think of it. There is only the necessary, and choosing is pure fantasy. Without true moral agency, there IS no existence as a moral agent–there is only reflexive and metaphysically necessary reflex action.
In short, without moral agency there is ONLY “meat-sack morality.” You act in ways that are pre-determined, just as any meat-sack would! THAT is the logically necessary end of a philosophy of biological reductionism.
And “complexity” in no way solves the problem–this is a foundational problem! Complex machines are no more intrinsically agentive than simple ones.