You can run Mastodon in a decentralized manner (everyone running their own instance), but most users do not want the drawbacks of such a setup, hence the current Fediverse having nearly all users on multi-user instances of Mastodon, Pleroma, etc.

Sure, those are federated, but unless your running your own server ala Brier then it's not decentralized.

Most people do not need full decentralization though, a trusted instance run by a competent administration is sufficient. Burning battery and data to run a server on a mobile device is not optimal.