Because a shared design system’s components get reused across dozens or hundreds of pages, fixing accessibility at the component level, correct focus states, proper ARIA on a custom dropdown, sufficient color contrast in every color token, has an outsized impact compared with fixing the same issue page by page.
It also flips the usual dynamic: instead of accessibility being a late-stage audit finding, teams building new features from an accessible design system get much of it right automatically, which is one of the most effective long-term investments an organization with a large or growing site can make.