^Because it's needlessly literal-minded and missing the point. The makers of the episodes did not intend every frame of film to look exactly the way it did. They were operating on a tight budget and schedule, with limited technology, and so they had to make compromises. They had to settle for something less than what they wanted, because that's what you do in television. They wouldn't have wanted the audience to take every frame as exact, undeniable gospel. They would've wanted and expected their audience to think for themselves and interpret what they were being shown, to understand that it was an approximation and to use their own imaginations to bridge the gap between the limited, imperfect execution and the underlying concepts it was supposed to represent.
Reading a text purely on a literal level, taking every glitch and error as absolute gospel, is lazy, mindless, and slavish. It's not the way any intelligent person interprets a text, and it's not the way the creators of a text want you to interpret it. If I write a novel, say, and fail to correct a major continuity error -- say, I set a particular ongoing sequence on Tuesday in one chapter, Wednesday in the next, and back to Tuesday for the rest of the book -- I don't want my readers to assume that's what I actually meant and concoct some insanely overcomplicated rationalization for how the whole cast jumped forward in time briefly and back again. I want my audience to have the basic common sense to recognize that I made a mistake, and to correct that mistake in their own minds and thus get my actual intent that the whole thing took place on Tuesday.
It was not the intent of ST's creators that the ship was magically jumping back and forth between three or four different configurations in the blink of an eye depending on whether they were using footage of the 11-footer, footage of the 3-footer, or stock footage from the pilot. It was not their intent that so many planets had identical continents. It was their intent to use the limited resources at their disposal to suggest the ideas they really wanted to convey and trust that the audience was intelligent and imaginative enough to look beyond the mere letter of the text and perceive the underlying concepts.