Your question could be rephrased: why do so many projects depend on an external store for distributed consensus? One answer is that coupling the consensus part

Your question could be rephrased: why do so many projects depend on an external store for distributed consensus? One answer is that coupling the consensus part of the system with parts that do active work results in harmful resource conflicts. Those resource conflicts can cause consensus algorithms to fail or take much longer to return answers. Example: Java VM clocks misbehave if the process can't get enough CPU. This can cause systems like ZK to lose quorum.