Tradition. In early times you were only able to signal one bit at a time on one wire. At that time there were also no consensus on the size of a byte in bits.

Fun fact: 1kbit/s is always 1000 bits per second and never 1024 bits per second.

fun fact: and this is why you need to divide by 8 to get theoretical speed, or use mental shortcut of dividing by 10 to approximate protocol overhead.

different layers. Wifi is always marketed using scammy layer 1 speed, this is why your 54G never went above 1/2 claimed speed.