First, being pedantic, you mean the conspiracy theories surrounding them. Certainly there are some crazy theories. At the same time, given the suspicious circumstances of JFK's killing, there's a decent chance a conspiracy to kill him between some people did occur. With secret government technology... I don't know if that counts as conspiring. We know they develop things in secret. Of course, you can have crazy theories about what they're developing. It's also, in either case, crazy to believe a theory is true without any evidence.
Loony, completely unsubstantiated fringe deviations aside, governments really do develop and build secret technology in secrecy, in ways that can only be guessed at by very hazy inference for a time. And there are enough weird things about the JFK assassination for it to be understandable that many people, even serious investigators, spent decades with doubts about the basic lone gunman narrative.
