It seems kind of silly to ask if anyone has thought of making new parts, but telling them not to mention the company that does. Pretty self defeating.
Ignoring that, the reason that you can get replacement parts for a retro car and not for a retro computer is cost. The setup is the biggest cost. If there were as many Amiga owners needing replacement door panels as VW Beetle owners needing replacement door panels, and those Amiga owners were willing to spend as much for a door panel as a VW Beetle owner is willing to pay for a door panel, you would see replacement panels panels made available.