The companies do so, because they want to ensure that the customers buy their products again and again. If the product is of utmost quality, then the needs of the customer would be satisfied with the first purchase only. This would mean that there would be no repurchase, which would not be good for the company.
For Example: If Fair and lovely is manufactured to make the customer fairer by 4 shades in 4 weeks, then the customer will not purchase the product again. Similarly, if a new is produced such that only a single granule of tea is sufficient of 2 cups of tea, the volumes of tea sold would be very less.