Why are so many...

Large format cameras, especially 5x7 and 8x10, missing the backs? I've notice that many LF cameras for sale on the net either have no back or the back is a reducing or split frame back. I would think that the focusing glass might be broken or missing but the entire back?

What is your guess?

Some might have been made for plates, not sheet film. So those backs probably go lost as plates lost out to sheet film decades ago. Others probably had their backs taken off and sold years ago, as 4x5 cameras are a lot more common than larger sizes. So at some point someone was building or repairing a 5x7 or 8x10 and said, "I'll give you some money for that spring back", and they took it because they didn't think the camera was worth all that much anyway. Others still probably were converted to 4x5 because as the price and quality of film went up, the larger sizes didn't make as much sense unless you're contact printing (or these days, scanning). Those are my guesses.