The whole megabits/gigabits thing seems like a holdover from when internet speed was measured in bits. Not kilobits, but bits. 300 baud (bits/sec) modems and the like. When 1200 baud was reached, no one wanted to be the one to switch to a “lower” number of ~150 bytes/sec.

EDIT: baud is not exactly the same as bits/second. But in those times, it was pretty much the same. https://stackoverflow.com/questions/20534417/what-is-the-dif...