A couple of posters have come close to the real point: Why have such a high proportion of the Star Trek cast, relative to other TV shows, gone into directing?
Because they are so typecast they can’t get any other work.
After the classic series went off the air, almost none of them could get another acting job. Nimoy managed to get on Mission Impossible, and Shatner bombed in Barbary Coast before finally getting regular work in T.J. Hooker (notable for one of the first broadcast fart jokes), but the rest of them were stuck working the conventions for years. The movie Galaxy Quest depicted their plight fairly accurately.
Actually many tv series let their actors direct an episode. I’m pretty sure many of the Friends cast has directed and I know MASH had lots of episodes directed by the actors.
The big reason…
TV is a PRODUCERS medium. The TV director can’t do that much to change things. Especially if the cast is playing a role they have played for years so the actors know what they’re doing. TV does not have quite the inventive camera work that a film would have. So when an actor wants a new creative challenge, a slightly bigger paycheck, or some ego boosting, they can direct an episode and start cutting their director’s teeth on something not that difficult.
Some do it once and never again. Some go on to great directing careers.
Amanda Bearse started directing episodes of MARRIED WITH CHILDREN in her mid-thirties, as she noted that roles for character actresses (at least gamines like her) start drying up then. She’s quite good, with a deft touch and a knack for music and rhythm, and has directed lots of other sitcoms since. All the MWC actors, at least, said they’d do their best for Amanda, which might be another thing–if the actor’s popular, the cast will listen to them more closely.
That’s a bigger jump than director, I think, but a bunch of them have done it. Nimoy has a couple of books out. George Takei co-authored a book about a futuristic ninja with Robert Lynn Asprin (I think he’s still obsessed with that fencer’s role from the old series), Walter Koenig has at least two books out. And there are a lot of books with Shatner’s name on them.
