Might as well paste my answer here as well.

It’s because it used radio frequency. Yes through a cable. The cable was basically just an antenna. The RF box in the back of your console or whatever would send the RF signal at a specific frequency, usually channel 3 but some had switches that you could change to channel 2 or 4.

It’s basically like tuning in to a radio station.

Edit: That's also why you'd get static when running a microwave, vacuum or whatever. The TV would pickup the noise generated by those machine.

Edit 2: As another user has mentioned, the option to switch the channel existed in case one channel had interference issues.