While CBS quit this process in September 1972 and ABC stopped around January 1978, NBC continued until Fred Silverman ended it in 1980. As such, some companies and networks either destroyed the tapes outright or reused them for other shows in a cost-saving measure, typically resulting in the disappearance of many shows and (in)famous moments. For those shows which aired live, their nonexistence today is usually due to simply not recording it for videotaped shows, especially in the 1960s-70s, the 2' quadruplex videotapes were bulky and expensive to store.
In the years before home video recording became mainstream and videotapes were rather costly, most broadcast networks and production companies generally saw television as one-time programming.
