In almost every old fixture, the metal outlives the glass, which is why a replacement glass shade is what so many restorations need. Steel, brass, and bronze bend, corrode, and tarnish, but they survive; glass shatters, and once it's gone it's gone. That simple fact shapes the whole salvage of lighting, and it explains why intact original glass is so often the rarest and most valuable part of a fixture.
The reasons are physical. Glass is brittle, and a fixture takes knocks across a century, in service, in removal, in storage, in shipping. A metal body survives a drop that destroys the shade it carried. So glass survives at a lower rate than the metal around it, and each surviving shade cleared more hazards than its fixture did. This inverts what you might expect about value. The fixture is the larger, more visible object, but the glass often determines whether a piece is worth having, because a sound fixture is common and its matching original shade is not. A fixture missing its glass is a body waiting for a shade; a rare original shade can be worth more than the fixture it sat on. It's also why so much of the trade is in glass specifically: replacement shades, in the right size and style, are what a great many restorations need, since the fixture survived and the glass didn't.
Salvager's Note: A fixture missing its glass is common and cheap; the right original shade to complete it is the scarce part. When you find an intact period shade in the size you need, it's usually worth grabbing, because the fixture will wait and the glass won't reappear.