He's not an actor. He's a star
Really, he long ago decided to play himself playing an actor playing himself. "Free Enterprise" was the best thing that every happened to Shatner. He completely redefined himself.
As for his earlier work, it is absolutely in the style of every other drama on TV at the time. Check out "Bonanza" or "The Big Valley." Almost all shows were broadcast and then rebroadcast only once. That was the expectation, anyway. There was no time for subtleties. The audience needed to be told fast what the story was about.
Is it often hammy now? Sure. But no one, even in the first decade of syndication, ever, ever complained about it. I lived through it all, and while fans complained about certain guests, no one ever criticized the regular cast's performances. Back in the '70s, the most common complaint about Shatner in "Star Trek" was that he was sometimes "fat." (Which I just do not see in TOS. Maybe a few extra pounds, but never fat.)