Why MongoDB needs to disable majority read concern for PSA architecture?
Arbiter nodes don't "replicate" data...

The problem with your setup is that, that you can "lose" two of your three data-bearing nodes and your setup has still "majority" in votes. Meaning that replica set still keeps "serving" clients, but all data is only at the one node without replication.

Then comes that "majority RC", with that setup, even you lose one of those data-bearing nodes, the whole replica set stops accepting data (basically) because of missing ack messages. If three nodes of five are data-bearing nodes, that three IS majority of nodes and "majority RC" required that all those three nodes answer "ack, data is safely saved to the storage".