There is whole raft of different approaches to the handling of spares.
As Lutz points out an item like a glow plug, is small, and it may be used on multiple vehicles, so it could still be a current production item, and for many mechanical parts some companies might have a number of suppliers from which to purchase. This will tend to keep spares prices and availability good.
Items like body panels are bulky, and if they are not for present production, any stock will have a high storage cost. If the panel is a self coloured moulding, then what colours should they keep? the scale of the problem increases with the numbers of different colour ways they offered. Often small changes are made to the panels over the run of a vehicle to accommodate new light fittings or trim items.
Its more likely the tooling will have been kept (usually by the OEM supplier or an dedicated aftermarket supplier) and if a piece is needed then it has to be moulded specially. This is made more difficult as the moulding compounds have to be sourced and mixed to get the required colour etc.
Keeping spares (especially for products no longer in production) is a very costly business. There are additional administration costs, and it is legitimate for businesses to make a profit on spares. However I do know some business do over inflate the price of spares items simple becasue they can.
Generally though its is still cheaper to effect a repair than to scrap and buy a whole new car or caravan.