I think congestion is the main reason. If a transmission had unlimited range (hypothetically), the band would fill up pretty quickly, so it would stop being useful. Natural limits of signal transmission and power limits are what makes public, unlicensed bands feasible. They limit the possible uses, but, stochastically, enables many people to use it at the same time.

At any repeater hop, the transmission uses double the airtime in the overlapping of the sender's and repeater's ranges (once while the sender is sending it, once while the repeater is repeating). And of course, if you use repeaters, a single transmission will use the band in a much larger area than a direct transmission.

With a mesh network, a product like this would be very problematic: From "a way to connect with friends while off the grid" it would turn into "unlimited free texts for a one time charge" (in a country with absurd charges for texting on mobile phones). Soon, neither this nor any other application on the band would be possible anymore.