If you send 1 bit simultaneously you need two different levels (for amplitude modulation). If you want to send 8 bits simultaneously you need 256 levels, which will result in a lot of read errors; a level may change due to network attenuation or noise.
There are ways to send more than one bit simultaneously however, like QAM (Quadrature Amplitude Modulation). Part of the information is in the amplitude of the signal, like in ASK (Amplitude Shift Keying), and part is in the phase of the signal, like in PSK (Phase Shift Keying).

If you send 1 bit simultaneously you need two different levels (for amplitude modulation). If you want to send 8 bits simultaneously you need 256 levels, which will result in a lot of read errors; a level may change due to noise.
There are ways to send more than one bit simultaneously however, like QAM (Quadrature Amplitude Modulation). Part of the information is in the amplitude of the signal, like in ASK (Amplitude Shift Keying), and part is in the phase of the signal, like in PSK (Phase Shift Keying).