> Each one adds significantly more bandwidth (but worse propagation for a given transmitter power).
As far as I know, that's mostly due to these bands simply having more spectrum available, in addition to actual physical propagation characteristics (lower frequencies indeed propagate a bit better across obstacles like walls or floors, and slightly better through air, but not at all through free space).
In addition to that, unlicensed bands arguably work precisely because they are (by regulation) limited mostly to short-range applications, using maximum transmission power as a proxy for range. One person's signal is another person's noise, after all.
There's other unlicensed bands available that are more suitable for long-range communication, but these usually come with duty cycle restrictions for the same reason.
As far as I know, that's mostly due to these bands simply having more spectrum available, in addition to actual physical propagation characteristics (lower frequencies indeed propagate a bit better across obstacles like walls or floors, and slightly better through air, but not at all through free space).
In addition to that, unlicensed bands arguably work precisely because they are (by regulation) limited mostly to short-range applications, using maximum transmission power as a proxy for range. One person's signal is another person's noise, after all.
There's other unlicensed bands available that are more suitable for long-range communication, but these usually come with duty cycle restrictions for the same reason.