For backward compatibility with PATA/ATA/ISA, SATA uses the PATA/ATA/ISA command set. PATA/ATA/ISA is simplex because it is a connector to the ATA/ISA/PC/XT bus which is simplex.
The ATA/ISA/PC/XT bus was simplex because it was a connector to the 8086/8088/80286 processor data bus, which was simplex.
Q: Why isn’t there a wire on pin 20 (POWER) on the standard external DisplayPort cables?
A: The reason power isn’t included in standard cables is because both source and sink devices are designed to provide power. Captive, attached cables often include the power wire. If it is desired, for example, that a particular source device utilize the power available from the mating sink device, then that Source device could include an attached or dedicated cable that carries the DisplayPort power signal. Same could be applied to a sink device.
Since Displayports allow for daisy chaining and other features, sometimes its good for a sink device (like the monitor) to provide power. Sometimes it is not. A direct connection between a Source (like your Computer) and the Sink device is not a situation where both power pins should be connected/used.