A large part of the problem is that people, average people, don't look up into the sky very often. This is why things like bright stars, planets, and even the moon, are sometimes mistaken for UFO's.

The same is true of "chemtrails". Many people don't really understand what contrails are, how they are formed, or how they can persist. So when they see them they are immediately wary. They also don't understand just how many planes are in the sky at any given time. So when some lunatic starts spouting off fairy tales about government/corporate attacks on the population, people get scared. Everyone seems willing to accept that the government would do something like this, without bothering to figure out WHY they would, or at least why they would do it in such an obvious and haphazard way. There are easier, cheaper and far more effective ways to kill off your population, after all.