Porridge requires boiling water, which requires a heat-proof container, which was rare before the invention of pottery. There are ways to boil water without pottery (dig a pit, waterproof it with hides, fill with water and drop hot stones in it, for example), but they involve a fair amount of work.

In contrast, grinding stones are very old in the archaeological record, predating the invention of pottery by tens of thousands of years. You don't actually have to grind grains all the way down to a fine flour, it's enough to make a coarse flour.

Baking bread is very simple. Moisten the coarse flour with water to make a paste, spread it on a flat stone and set it by the campfire to get basic flatbread. People were doing this 20,000 - 30,000 years ago, long before agriculture and pottery.