Computer networks send data one bit at a time. It’s a state change, either electrical or with light pulses. It happens faster and faster as technology improves, but it’s still just one bit at a time. Hence, bits per second.

It doesn’t matter whether it’s a file or a digitized real-time data stream or a bunch of random characters, it’s sending one bit at a time.

Also, when you’re transmitting a file over a network, it’s not just the file that’s being sent. Overhead bits are added to the stream, whether it’s the source & destination addresses, fragmentation flags, error checking bits, and a slew of other bits used for controlling the traffic somehow.