Oh, I'm sure it's cheaper to operate, otherwise they wouldn't do it. But I am of the opinion that energy and many natural resources are extremely underpriced when one factors in climate change and other externalities like the impact of exported e-waste.

Would you need to compare it to the environmental impact of the lifestyles of the dozens / hundreds of human workers who would be staffing a traditional warehouse?

Do the humans only exist to staff the warehouse?

No, but they have to drive or otherwise travel to the warehouse to work, provisions have to be made for them, such as air conditioning, and so on.

A human at work is hardly environmentally neutral.