Because we made the algorithms and can confirm these theories apply to them.
We can speculate they apply to certain models of slices of human behaviour based on our vague understanding of how we work, but not nearly to the same degree.
Hang on, but- plagiarism is a copyright violation, and that passes through the human brain.
When a human looks at a picture and then creates a duplicate, even from memory, we consider that a copyright violation. But when a human looks at a picture and then paints something in the style of that picture, we don't consider that a copyright violation. However we don't know how the brain does it in either case.
How is this different to Stable Diffusion imitating artists?
We can speculate they apply to certain models of slices of human behaviour based on our vague understanding of how we work, but not nearly to the same degree.