Data is send serially over a fiber or copper line, bit by bit, not per byte. Also because bytes can be compressed, bits cannot, so giving the throughput in bits-per-second is an absolute value.
Bytes can be compressed in sequence not in singularity (just as bits). I mean why don't we measure storage in bits and not bytes? Just seems unnecessarily confusing.
Actually, memory chips (NAND flash, DRAM) are usually specified in terms of bits, usually using a lowercase 'b' to denote 'bit'.
Online sellers have used this to their advantage, selling e.g. "1Gb" flash drives, with real 1 gigabit flash chips, which turn out to be ~128 megabytes.