An interesting question I myself never thought of also ... my guess would be the Ethernet cable have limitation on the distances. Hopefully someone here could give a more proper and technical explaination.
My teach always said, there is never a stupid question.Just question never asked and remain clueless
Ethernet had a maximum length of 100 m. Originally long distance communication was a analogue rather than digital. The serial cable had a series of pins for data and clocks. Eventually we had digital circuits. Finally we delevoped methods of carrying Ethernet over these digital circuits.
So now you can connect routers using Ethernet but technology cycles can be quite long.