@strongsand I'm not sure but possibly because the protocol on the physical layer transmits one bit at a time and doesn't care about the size of a byte (plus bigger numbers looking more impressive which was already mentioned) (I don't actually know the details of how data is transmitted though, but ethernet is definitely serial)
sizes of SPI NOR flash are also measured in bits for some reason, which is confusing because the sizes of the stuff that's stored on them (at least when they're used for storing boot firmware) is measured in bytes