star - Why can't this be the simple (and obvious) explanation for the dimming of KIC 8462852?

As others have said, there is repeated dimming of the star, that suggests something in orbit around the star. But in addition to that, you are having the perfectly reasonable and common failure to comprehend the vastness and emptiness of space.

Now, comets, planets and the like are quite common going around stars, but in interstellar space they are very rare and to line up exactly with a distant star is incredibly unlikely. To get an idea of the scale, think of the shadow cast by a flea in London, from a light bulb in Beijing, being seen in New York. It just doesn't happen. Because the distances are too vast (even this example is wrong, by a factor of about 1000). Space is just too big and too empty for random debris to exactly line up with a star.

Now if the debris is near the Sun, as is passes in front of a star, the light from that star is abruptly cut out. This is called an occulation, and they happen very often. The conclusion is that whatever is causing the dimming of this star must be close to it. Probably a cloud of dust in orbit around the star, but this doesn't explain how that dust got there, and why it doesn't appear in the infrared.