At a fundamental level the intended meaning of "federate" is that disparate communities and softwares can "connect" seamlessly, a bridge by definition is a tool used to connect things that are not connected or seamless.

A federated landscape of interconnected trails is the structural antithesis to a landscape of bridges each laboriously muscled out of the headache inducing process of connecting two disparate systems with a third system specific to that bridge and that bridge only that must be endlessly revised and rebuilt to keep everything from collapsing in a heap.

A bridge, by definition, is a composite of parts that are existentially vital to the sucessful conveyance of what passes over them, it only takes one section failing to break the entire bridge and it only takes one troll to block everything. A bridge, again by its very definition, is the most brittle architecture as every bridge is ultimately only a temporarily open door that must be continously be maintained and eventually rebuilt at the expense of great effort.