This reflects my own experience too. Generally speaking, incompetent people don't get promoted in the same way you normally understand promotion. They don't get their boss's job. They don't stay in their boss's boss's reporting line.

They move to a nominally higher level role in a different department, often a cost centre, generally to some role which serves the organisation just by existing (e.g. a legally required role, or something that serves some certification purpose, or so that sales teams can say 'we have a dedicated person in our X office for doing that').