Why Do Background Removers Always Make Fake-Looking Edges?

https://squareblogs.net/viliagmcoi/h1-b-case-study-how-clean-edge-detection-fixed-a-background-remover-that

Why this list matters: get real-looking edges without endless fiddling You’ve used every background remover out there and ended up with subjects that look pasted on - thin halos, unnaturally smooth hair, or hard, jagged outlines