I stock shelves and help manage inventory at Wal-Mart, and I can tell you why.
Bread is usually subsidized from a local company. The store I work at buys bread from one of the local bakeries, Nickels Bakery. All the Great Value brand bread is actually Nickels' bread in a GV package. And it's about $0.15-$0.20 cheaper per loaf. And it always rotated by the vendor every day.
Low-end cheese is basically all the same. Most of it isn't even real cheese, but cheese "byproduct"... whatever that means.
Great Value snacks actually are Hostess brand, just repackaged and subsidized to make it cheaper.
Most Great Value brand food products are simply subsidized and repackaged products from name brands, packaged in a factory that doesn't have a union.