My understanding is that they just piggyback on the nearest Bluetooth-enabled Apple device, which then pings Apple's servers; non-Apple-specific equivalents like Tile work the same way (just with different host-device-level software and different centralized servers).

That said, Bluetooth mesh networks are absolutely a thing (https://en.wikipedia.org/wiki/Bluetooth_mesh_networking), and it'd be neat if low-power devices could capitalize on that.