It's not just about the frequency. It's about the modulation. You need new/additional radio hardware. Constantly being on another band eats more battery, too.

But as we've seen in busy areas, there's just not enough bandwidth with many nodes on LoRa. It'd get congested real fast if it were built-in to mainstream phones.

Using the hardware that's there, something like briar let's you get your messages routed through whichever means. Bluetooth mesh, internet or tor.