Since SATA uses 8b/10b encoding, the data rate in bytes/s is actually 1/10th of the raw bit rate. The same applies to earlier PCIe versions. 6Gbps SATA can transfer 600MB/s (ignoring protocol overhead).
I don't really know why it's become standard to publish raw bit rates in bit/s and data rates in bytes/s with error correction taken into account but not protocol overhead, but those are the two kinds of numbers you almost always see quoted nowadays. Raw bit rates at least map pretty directly to clock speed, and I guess protocol overhead must be too variable and too complicated for most people to bother explaining.
I don't really know why it's become standard to publish raw bit rates in bit/s and data rates in bytes/s with error correction taken into account but not protocol overhead, but those are the two kinds of numbers you almost always see quoted nowadays. Raw bit rates at least map pretty directly to clock speed, and I guess protocol overhead must be too variable and too complicated for most people to bother explaining.