Because prices are determined by supply and demand, not production costs.
Production costs directly relate to supply.
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Lower production costs go to higher profit, not (necessarily) increased production (which would shift the s curve and lower price), because these companies are likely already at their optimal output to maximize profit.
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https://en.wikipedia.org/wiki/Profi...e_product_of_labor.2C_and_profit_maximization
Price elasticity of demand for many goods is not linear, therefore lower prices can lead to correspondingly larger increases in demand. ie: cost declines per unit can allow a company to capture greater total revenues while maintaining the same costs.
This is pretty much the inverse of why people are wrong when they say companies pass all costs on to consumers. They may pass some of them on, but the demands of a competitive marketplace often limit the total amount that can be.