. . .As a kid, back in the 90s. . .
You're criticizing a television show made perhaps 20 or 30 years before you born because it doesn't meet modern standards? Are you able to watch a television show or motion picture if it was filmed in black and white?
When Star Trek first aired, they were essentially making up the universe as they went along with each episode. The only tv shows with story arcs were usually the daytime soap operas, if you can remember what those were. As others have mentioned, performance styles have changed in many areas of presentation. That doesn't mean something made in 1966 or 1936 or 1906 isn't worth your consideration.
And frankly, expecting TOS to hold up to a standard decades later established by its sequels, which built and expanded on its own universe, sounds rather ridiculous.