When you purchase a link from a normal website, people are aware what page their link will go on and what the benefits will be. In regards to directories people are blinded by the homepage ranking, even the strong directories have little weight to their internal pages. Most are not even indexed and nearly all of them have no optimisation.

Also a directory page will suffer because their is no unique content on their pages.

Websites that offer links for sale generally offer other services, what does a directory offer apart from PR?