I do not see any chemtrails when I look up at the sky where I live, in fact, I rarely see a contrail, unless the weather conditions are right for them to form. Planes fly over my area all day, everyday, but these things are not observed. If I did see something that shouldn't be there, or more to the point, what I think shouldn't be there, I would investigate it. Which is exactly what I am doing, with the claims of chemtrails.
"and if they deny it's existence, is that not enough?"
Who are "they"? How do you know that "they" are not telling the truth, and for what ever reason, the "others" are lying?
So no, if someone says something doesn't exist, that isn't enough for me to assume it does.