bedir than
Full Moon Storyteller
I think the OP is confusing a game rule, with how the game world might work. There is no direct correlation. The player rules are not a simulation of the game physics. You cannot infer the rest of existence from the PHB. Having a cantrip be unlimited does not necessarily mean that a caster can continually cast it, once every six seconds, forever. Maybe it does; that's for the DM (and players) to decide. What it does mean is that you are free to dispense with the minutia of tracking usages of cantrips. You've got enough of them for the foreseeable game day, however long that may be.
One could also do this kind of hand-wavey aspect that says "don't abuse the rules" without actually tracking it. Whatever suits your table and feel.