RDBMS replication and failover is way more difficult and manual than anyone would like. You can't just set up two postgres, tell them they're clustered and have it basically work; at a minimum you have to design the client to somehow know which one is currently the master, or use some sort of proxy (which becomes its own SPOF).

RDBMS integrity basically requires that one master server is responsible for the whole data set and other servers may replicate from it. And it usually doesn't wait for a quorum of replicas, just for one, because the design is to recover from a hardware failure, not a network partition, although that could be fixed at the cost of increased latency.