Because everything is measured in BYTES, but some "genius" salesman realized hey we can make the number x8 times bigger by saying megabits per second instead of megabytes like everyone else.

This is somewhat incorrect. Technically speaking in a binary system fundemental data is the bit. That is why in networking speed is measured as bits per second, because data is transfered exactly as like that. One bit at a time.

Using it in data storage though is an arbitrary choice.