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.

> If any store used dynamic pricing to expand their margins, the others would just do the same and compete away those margins once again, with the marginal gain being handed back to consumers.

How would you do it if pricing is dynamic and changes every day?

By the time competitor finds out about the price, you might have already reduced it, making it look like theirs is more expensive even after they applied discount.

Dynamic pricing based on personal data is not even a market, let alone a perfectly competitive one. Temporal dynamic pricing can mean almost anything, so might be ok (early bird lunch deal) or pure evil (bottled water now costs $100 because there is lead in the tap water).