Because most acids react with iron and carbon that make the steel, making it soluble and over time it would rust out the container. Most acids can be stored in Teflon coated steel containers though, and even glass gets etched by acids over time.
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As acids react with metals like sodium potassium magnesium zinc making all off them soluble in water and they gets easily rusted and corroded by them. they also  liberate hydrogen gas. industries use carbonised steel to store acids and also in bottles made of teflon, polyethene polymethylpentene( type of plastics), glass.