Every channel draws its frame differently. Amazon wants the product to fill most of a pure white main image, fashion marketplaces often use a tall 3:4 portrait frame, and your own store follows its theme. One crop rarely suits them all, and a careless resize cuts off feet or leaves the product floating in space.
Doing it by hand means a designer opening every image for every channel. Across a catalogue that is thousands of exports, and every new channel adds another round. The white-background entry covers Amazon's main-image rules in detail.
