The original Ethernet LANs ran on coaxial cable and used baseband signaling. Only one frame can be in flight across a baseband coax medium at any time. Just as a car pulling onto a road must wait for a break in the traffic before entering, the CSMA/CD rules require a station to wait until the medium is free before it can send another frame. Stations share the medium in a (more or less) fair manner.
However, when LANs are constructed using twisted-pair or optical fiber media connected to switches, the CSMA/CD rulebook can be thrown away. Data can flow from station to station, station to switch, or switch to switch in full-duplex mode.