Manufacturers are not interest in maximising their social benefit: they want to maximise their profit and this is done by producing fewer items but at a higher price (to be technical, a monopoly would produce until marginal revenue = marginal cost).

Manufacturers can set recommended retail prices but their are limitations to how these can be enforced.

> Why do manufacturers sometimes set retail pricing

depending on how it is set - most manufacturers set a recommended price, rather than contractually obligate the retailer to sell at a particular price. It's an attempt at price-fixing (without it being price-fixing), but other manufacturers of the same goods could choose to lower their price for competition. It's not the same as price-control.