This is all thanks to ABC archiving everything, not an employee saving a tape. However that does happen sometimes at other networks.

ABC and WGBH/PBS are some of the better ones you can find almost full digitized archives of online. Other broadcasters like NBC didn't save tapes until 1980 (multiple Late Night with Letterman episode masters being lost as an example). CNN is another somewhat bad one, they literally threw all the masters to their theme music in the dumpster, but an employee saved them all. They have an internal archive of tapes as well as the "CNN Collection" but it's spotty.