The answer is simple: if you do a auto-failover against 2 nodes, you don’t have a replica anymore. The cluster does not want to put you into a bad state without you manually performing that operation. Note that you can do manual failover on two nodes.
Other than that, note that we strongly recommend to run at least 3 nodes in production, for the exact same reason as above. If you have 3 nodes and one fails, you can still failover & rebalance and be back at one replica.