Any intriguing event of such global importance is going to attract all sorts of theories, many of which are going to be unfounded or highly improbable, such as Paul MacCartney being replaced, crop circles being made by aliens, the earth being a flat disc, etc. With JFK you get shot by the driver (because on a poor copy of the Zapruder film the top of a head looks a bit like a pistol), shot by accident by AR15 in following car (because a photo shows the gun draw, but this happened after the shooting), shot by aliens, no doubt! Because people want answers in the absence of certainty, and people are happy to write books to make money, and it all sounds intriguing.
Most humans seem to have the need to believe; you only need look at the preponderance of cults and world religions.
It appears to be quite probable that there was a single shooter, probably Oswald. Evidence of cover-up suggests there was something else to hide and a number of key figures had opportunity, means and some strong motives.