Here's why I don't reply to mine, and I guess it's most similar to your reason #5 (about being trolled out).
Basically, I don't reply or even read any of my comments because, I should probably mention that I have pretty severe Social Anxiety. For me, this transcends in-person interactions because it has to do with my thoughts and concerns with what others are thinking.
I always anticipate there to be rude comments, and in my mind, even one or two comments are too overwhelming to have to deal with. To imagine a situation where I have to defend myself against an attack, accusation, of just something incorrect (or rude) ends up something I would rather just not even know about (i.e., I do feel better just imagining that my comments are all nice rather than reading them and finding out there's now a list of mean messages I have to address).
The funny thing is, from the beginning I've always told myself when I get more subscribers (that means "fans" right?) I'll then have the courage to look and respond, but there's a part of me that also knows that's unlikely to happen.
A much smaller reason I don't respond is because I don't like social media, talking to people via social media, etc. It is not just not enjoyable to me, but it seems tedious. I work very hard on my videos as is. I mean seriously, I go overboard most often. This is something I'm newly realizing just now but the way I feel is like I've basically already said all that I wanted to say in my video--including giving appreciation out to my subscribers and viewers. I don't think creators should feel obligated to cross the boundary from creator to "friend". What is there to say to people in comments? You can't respond to everyone individually and my OCD would make me crazy over that aspect too. I probably WOULD respond to every comment, and that would take a very long time.
I know this is an old thread, but I replied because the topic still seems relevant.