Why can they do this? One reason is monopoly power- monopolies must be broken up.
But what else can be done (besides central bank driven demand destruction)? So instead of price controls, a more palatable solution is collective buying. This gives buyers more clout, allows them to bargain for lower prices.
https://en.wikipedia.org/wiki/Buyers_club
So we need more large non-profit buyers clubs, things like that. In the US, allow medicare to bargain with drug companies- it's insane that they can not.
An example where this works: my parents live in a gated community. They pay very little for the cable + internet + cell phone because the entire community has a deal with the provider.
Large retailers have this exact role: they certainly demand and get lower prices from their suppliers. We need a non-profit version of this, why should Walmart owners only benefit from their buying power?