Honest answer (and thanks for asking):
First, I want to make it clear that I don't think of "climate deniers" as a group per se like a race, creed, or nation. There's a lot of detail and nuance in the world and I want to explore that, not abrogate it.
I just want to elucidate on that a little before I get to answering your question.
You have folks like the people in the oil companies who knew about the science of the greenhouse effect and still spent lots of money casting doubt on it in public to protect their profits.
You also have people who have honest doubts and questions. I'm even one of them. I've heard that global warming and climate change might have more to do with changes in the Sun's energy output than human chemical emissions, and I'd like to talk more about that, but ooo yeah not in public, eh? You would have to be all like "but I'm not a climate denial-ist" like OP.
So, just to repeat, I'm not being tribal here. (For instance the Chinese Communist Party creeps me out, but I still give them credit for e.g. fixing the Loess Plateau, etc. https://news.ycombinator.com/item?id=20537409 ) I'm not against a group, I'm sick of certain behaviors.
To answer your question "why [am I], personally, sick of [certain behaviors]?" It's simple.
When I was a kid in high school I found my parents' copy of the Next Whole Earth Catalog which was a kind of hippie how-to guide from the 70's. I have no idea what happened to the people who made and bought and read this thing. My parents didn't talk about it or anything like that. But it became, for awhile, something like my bible. I left my textbooks in my locker and carried it around instead (it's like four feet wide and weighs six or seven hundred pounds!)
In it I learned about people like Christopher Alexander, Bucky Fuller, and Bill Mollison, and I began to see, or think that I saw, a better world. A world where humans and nature were in harmony, and "no one had to get nailed to anything."
That was about thirty-five years ago.
So, from my POV, everybody has been running around enacting "Idiocracy" instead and, yeah, I'm sick of it.