Bandwidth isn't literally width like a pipe of water. It is linespeed. That is why higher quality cables have higher bandwidth, even though packet transmission is completely serial. "wdth" is an illusion created by time division multiplexing, like multitasking/multithreading on a PC.
This isn't always true because time isn't the only multiplexing strategy used. For instance, modern cable modems use code-division multiplexing, and fiber optic communications typically uses a variant of wavelength-division multiplexing. In both of the later cases, there is actually simultaneous transmission.
