As the constitution clause states, it's not the goal to have these exclusive rights; these exclusive rights are the means(motivation) by which to facilitate the creation and publication of such works.
The constitution is not a guideline to unlimited powers, but constraints to limited powers. If congress is passing various copyright laws for reasons other than to "promote the progress of science and useful arts" then it would be unconstitutional. In other words passing laws to enhance profits rather than to ensure creator motivation would be unconstitutional, yet that's unambiguously the motivation for nearly all modern copyright law. Nobody is deterred from creating a work because they would only own the rights to it for e.g. 30 years, let alone 110 years.