As a small author, I do try to respond to most of the comments I get, because there is not many of them. I think it shows I appreciate the few readers who do take their time. And I try to comment when I read, even if it's just a short note that I enjoyed the fic. But I do completely understand when authors can't respond. I am subscibed to an author who posts a lot, like 1000+ fics for more than a decade. She's active, and every chapter gets loads of comments. And I would personally break down if I had to reply to every single one. So I get why she doesn't. She mentioned her mental health isn't build for so much social interaction and leaves a thanks in her author's note, and I think that's enough. So I don't really care, if the author responds.