> Couldn't you send data at megabits per seconds over a mile long copper wire

Yes, but you need the bare copper wire without signaling. We operated a local ISP in the 90's and did exactly that by ordering so-called "alarm circuits" from the telco (with no dial tone) and placed a copper T1 CSU on each end. We marketed it as "metro T1" and undercut traditional T1 pricing by a huge margin with great success to the surrounding downtown area.