With the arrival of Disney Plus and Apple TV Plus — and with HBO Max and NBC’s Peacock coming next year — we’re nearing a time when almost every major media conglomerate will have its own subscription streaming service. But for a variety of reasons, some beloved TV series remain locked away.

Often, the original contracts for a show were structured to cover broadcast rights and syndication but didn’t factor in the home video market, which makes paying all the co-owners for a streaming deal too complicated and expensive. Or sometimes a single company owns the rights, but holds onto them in order to maximize their value.

If you want to watch a classic episode of David Letterman’s “Late Night” or “The Late Show” right now, for example, you have to rely on illegal uploads. The comedian’s company Worldwide Pants owns most of that material, and its stewards have announced no plans (yet) to license any of it to Netflix or Hulu. The influential Canadian sketch comedy series “SCTV” has also all but disappeared, because of music rights issues, aside from a Martin Scorsese-directed Netflix documentary about the 2018 cast reunion (which has no announced release date).