It is important to understand that the methods selected for the “Final Solution” were intended to minimize costs. It is dangerous for crime at this scale to become cheap.
The experience of the Einsatzgruppen in Poland showed that mass execution with guns used too much material and was too stressful for the perpetrators. Camps allowed operational costs to be amortized while individual responsibility was diffused.
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Sibling commenter is hellbanned, but relevant.