Perhaps my explanation isn't perfect, but... Maybe in part because it is unthinkable for any "grown-up", "responsible", "engaged" estimator in charge of the pro

Perhaps my explanation isn't perfect, but... Maybe in part because it is unthinkable for any "grown-up", "responsible", "engaged" estimator in charge of the project to put in 10x churn into the estimation of resources they request from their management? If you claim you would do that, imagine yourself justifying it to your manager, and then to senior management to which your manager drags you afterwards.