Not only that, but so many people are reluctant to pay for anything so your average installation is chock full of freemium plugins. I've worked on plenty of sites whose admin page looked a bit like the IE6 toolbar meme.
Unlike the free plugins, they're not reviewed by the WordPress.org team, and if you stop paying for them then you'll lose access to their future plugin updates, including critical security fixes.
I wouldn't say that their code quality is noticably higher, either; there have been countless CVEs for premium WordPress plugins over the years, and no shortage of discontinued/abandoned premium plugins that are no longer being maintained but are still installed on thousands of sites.