Why SATA is simplex even though it has separate TX and RX pairs?
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.