https://io9.gizmodo.com/a-brief-history-of-doctor-whos-missing-episodes-1835662666

Only 156 of 253 Doctor Who episodes from the 1960s survived in the BBC's archives. Many episodes are the victim of “junking”—the archival destruction of old videotapes and film recordings to make way for more modern programs recorded on modern formats.

Well haven't you answered your own question? Occasionally the odd episode of these classic television series is unearthed in a former employee's garden shed.

There are many good articles online regarding "wiping" of videotapes. There have been exciting finds, including cases of colour taken from an at-home video recording (!) of a show's episode -- where the encoded colour info can be restored to a new master of the programme, thought previously to exist only in b/w.

As for other series, some episodes of Dr. Who have been retrieved from overseas TV stations that preserved their copies -- which had been exported to them by the BBC. So sometimes the programmes find their way back home.:)