Decooney
1s And 0s with perfect square waves only exist in theory. A square wave requires an infinite bandwidth system that transitions from 0 to 1 in 0 seconds.
Since every circuit - even an infinitesimal trace of silicon on an integrated circuit contains a bit of impedance, capacitance and/or induction, they function like a passive filter. Which reduces the bandwidth of the cable.
When I was working on integrated circuits, we even had to account for how long the silicon traces were between different sections of silicon and simulate how the delays in the signals can introduce errors in the signal. While you can plan for error correction circuits for these delays/errors, they do seem to affect the reproduction.
This is what a 1 and 0 in a high speed HDMI cable looks like.
Recently I put in a cheap HDMI cable vs a decent but modestly priced HDMI cable and even though I was aware of the theory, I was still surprised the better cable produced a more vibrant picture.