Or more likely, laziness. The proper autopilot disconnect button is on the yoke, which requires moving around the 3D cockpit, or worse, re-enabling the yokes if they're hidden, whereas the disconnect bar is easily accessible on the MCP.
There's always mapping a command to a button or a key, but just about every advanced addon has its own custom command for this. You can script around it using e.g. FlyWithLua, but not everyone is willing to deal with X-Plane's "internals", and also what happens if you have a bug and the developer asks you to "remove all other plugins"?
The third solution is to remap the key/button whenever switching planes, but that's equally cumbersome…