Why Baseband refers to the original information signal before it is shifted up to a radio frequency, for example the roughly 300 Hz to 3000 Hz audio of a voice channel, or the video or data stream feeding a modulator. Modulation translates that baseband spectrum onto a carrier, and the detector in the receiver translates it back down to baseband. So the term describes the message signal's own frequency range, not anything about the RF carrier.
Watch out The choice about the lowest band the radio covers plays off the word "base" meaning lowest, but baseband is about where the modulating information lives, not about the tuning range of the equipment.
Baseband = the message at its birth frequency, before the carrier carries it anywhere.