What part? The Unifi controller software is not for "a device", it's for multiple devices, imagine having to manage SSIDs / auth for primary+guest Wifi network across just 5 access points. I don't think anyone would enjoy logging into 5 separate AP UIs. Add your Unifi gateway + switches to that and the need to have a unified control plane is pretty obvious.

If you want self-contained, that's a separate product line, usually prefixed with "Edge". What's "infuriating" about having two product lines I don't understand. Honda makes motorcycles, boat engines, cars and jets and nobody gets pissed about it.