One good reason might be more data ownership than tracking. Centralized platforms seem to at some point realize how valuable their user’s contributions to the site are, and change the deal eventually to e.g. lock things behind a paywall when they grow bold and/or can get away with it. Reddit and Twitter are two high-profile examples of this, but there are numerous examples from the past as well for things like nutrition tracking and cd cataloging. A federated model might better reflect the idea of community ownership, and prevent site owners from betraying the community and cashing out on their contributions.