Obviously because those humans then share and bear the moral dilemma of doing it or not. Which also affects the outcome, as opposed to just being some inconsequential detail. Humans can demand out of an injust war (as they've done time and again). Killing machines can not.

It's the difference between blindly following orders (which not even all Nazi's did) and being able to have a change of heart.

It's also the sheer (number of magnitudes) more killing efficiency of the killing algorithms, when a machine could kill 10000 people in the time it takes for a single human to make a decision about 1 of them -- until we find there's a != when we needed == in the algorithm.