Because no equipment can exceed the medium that connects it. A switch lasts five or six years, a server four or five, a laptop even less. The cabling stays. A latest-generation access point connected to an old, badly crimped or overlong cable is limited by that cable.
And cabling rarely fails obviously. It degrades. Slowdowns appear that come and go, illogical drop-outs, workstations always slower than their neighbours. The blame falls on the computer, the server or the carrier. Almost never on the cable.
The sum for the board: cabling weighs little in the total investment, but it is the component that lasts longest and is the most expensive to redo.