They were so stupid!
In all fairness, tape was expensive, so was re-used, agreements with unions, especially Equity, limited the number of repeats, and home VCRs, with the potential to create a market for selling copies to fans, did not yet exist, at least when the destruction began. Television was not seen, even by its own creators, as anything more than an ephemeral art form, and posterity was not considered.
With the knowledge of the world as it was at the time, the decision made good sense, much as fans like me might loathe it.
Years later, when VCRs were widely available, feature films being released on VHS, and the BBC starting to put out documentaries, they stated that it was unlikely that they would ever offer even classic dramas such as I, Claudius
to the public, as obtaining the necessary releases would, it was felt, be more work than the value of the market for such material.