> Were studios really that reckless with their properties?

Some were. Once the film made its money in the theaters it was then put in a vault and forgotten about. The theaters were supposed to return the release prints but sometimes the projectionist would "lose" them. The studio vault those films sat in sometimes catch fire or water leaks in. If the originals are destroyed then hopefully a few release prints are floating around in the hands of theaters, individuals (where those lost prints end up), or television stations. If not, then its gone forever.