At the end of the day, something has to be wired to the A/C to provide control and the majority of houses today are already equipped with that wiring inside the home (usually into a wall). I don't think there's any rule that says you have to have this in a wall, but I'd be willing to bet most people will prefer having that fancy smart thermostat mounted in the same place the old one was (myself included).

If nothing else, I think it creates a sense of security in that they'll be able to still use it if they lose their controller or their phone dies.