[pocketarc]

While I think there may be something to that, I think there's a simpler, more "it's just normal capitalism" explanation:

Making deals with every local small-time producer would be a big pain. Why do that when you can make a single deal with a giant monster of a tomato producer, and sell their tomatoes (especially since as the sibling comment points out, local tomatoes won't be as pretty and as uniform in quality)? There's economies of scale, prices are lower, the supply chain will be simpler, it's all better for the supermarket chain.

[nickpp]

Small grocery shops can and will deal with small producers. The fact we are losing both in favor of giant chains is largely due to regulation and subsidies.

Otherwise we’d have plenty of boutique retailers and producers charging more for better quality produce optimized for different factors.