I noticed this when we were over on the Naval Air Station. They have a bunch of different types of aircraft set up along the main entrance road, all of them have opaque canopies.
My theories:
It discourages people from climing on the planes to try to get a glimpse into the cockpit
The real canopy has been reused, being an easily removable and valuable piece of equipment.
Minimize sun damgage to whatever is in the cockpit
It’s just a gutted hull, and being able to see the hollow where the cockpit should be would destroy the illusion.
What’s the straight dope? Why do museum planes have opaque canopies?
You’ve pretty much answed your own question but #4 is the most likely. Nearly all the aircraft at Pima Air Museum have unpainted canopies save for those which still have the white “coccoon” coating aplied at the nearby AMARC facility. Fabric does get sun damaged over time but the interiors are doing remarkably well considering exposure to Arizona sun.
Aside from having high theft potential such as instruments, lots of the things in a military aircraft cockpit ar hazardous such as the seats as well as explosive bolts and detonating cord to detach the canopy. I’m sure they removed the ejection seats rather than “demilitarizing” them by removing the rocket motors and explosive devices.