This analysis is a bit shallow. Massive GSPs grew as a solution to deal with the massive problems that come with greens sourcing.

If an individual farm has an outbreak they can (usually) source it, eliminate it from the supply chain, and switch to an alternative without any interruption to service. That also means you want as large of a geographic distribution of your farm network as possible. (I'm looking at you, Yuma Valley - a semi-regular source of e.coli outbreaks).

As a bonus, the supply chain is much more efficient at this scale. Small farms can have their salads packed in nitrogen filled bags for longer shelf life and less waste.

Fresh leafy greens are naturally just one of the toughest and most vulnerable supply chains. The disappointing answer (like in a lot of these situations) is that being bigger and more massive company is what makes the product better.