Except that method would be more costly to the tax payer and unfair to someone whose conviction was found to be unsound and result in delays for other cases (you really don't want to have things like tribunals having to be done two or three times because the first one was done in haste, it tends to cause a backlog).
That's where we have to have faith in the justice system not screwing up cases! Granted...they kind of have a history there don't they?