That's the ideal dream scenario, but in reality the market isn't that efficient. Lots of markets gouge their customers and due to power imbalances the customers can't really do anything about it. The free market solution to this is just generally to let people suffer.
I don't see why consumers wouldn't just pick the stores that have better prices for them. Why would they know/care/need to know what the prices are doing for other people?
You can't see you mean? Consumers pick things that are reasonably convenient, achievable and known to them. These things are all subject to exploitative tactics by grocery stores.
There's also price-fixing, which famously occurred in Canada recently.
Not to mention cornering a market like Walmart would and removing consumer choice entirely.
All of these are separate issues from dynamic pricing
You were not asking about dynamic pricing. You were asking why you weren't able to see the reason that consumers don't always find & choose the cheapest option. The person you were responding to was explaining the unethical realities of food retailers. There's no reason why these would disappear with dynamic pricing.