Because a fluctuating pH is more harmful than some magic number. Plus those products usually use phophates which will lead to algae blooms (then they sell you anti algae products). There are much safer, stable and cheaper ways to adjust pH if needed but usually fish can adapt to most tap water conditions easily. Plus it is not the pH that is the issue but the kH and GH of the water (hardness) or the amount of solids dissolved in the water.