I think it's a matter of a low signal-to-noise ratio. So if you send one bit, but there's a low probability the other side will receive it, you'll need so much

I think it's a matter of a low signal-to-noise ratio. So if you send one bit, but there's a low probability the other side will receive it, you'll need so much redundancy and error correction to achieve the same bandwidth as a regular signal, that it ends up being at least the same transmission rate.