In short, because Personal Service Corporation ("PSC") rules virtually negate the tax advantages of having incorporated in the first place.
Moreover, few if any set up their corporation with the intention of being classified as a Personal Service Corporation and as such carry out certain activities which can be challenged on audit by the government. The audit can lead to very large fines, penalties, back taxes owing and interest. In extreme cases, it can lead to incarceration. An example provides some context.