They have to alter the packaging to account for the changes. They can either do so in a way that makes apparent to the customers that they're receiving less value for their money, making sure they're aware of it, or they can do so in a way that attempts to deceive consumers and hide this information.
An example of messaging for the former is described in the quote you quoted. Another would be to use different-looking packaging, to indicate that it is not what it was before. If a consumer will still buy the item when properly informed of the lower value, then this apparent labeling should not have any effect on sales. If it does, it means the information hiding was material, which makes it bad.
tl;dr: companies hide this information because being deceptive increases sales, if it didn't, they wouldn't
