For the simple reason that garage doors that fit in a listed building do not grow on trees, even if they are made of wood

, and cost far more than 6000:- if you are going to have them manufactured.
Using fittings from an existing door
can be an alternative, if the fittings are such that they can be removed and reused on a self-made door. But preferably I use fittings suitable for self-manufacturing, if they are available.