Authors can put the onus of commenting culture on ao3 on the readers, but there is, I think, a problem that they Really ignore. Ao3 authors dont reply to readers anymore.
When I was in the 2018 ao3 comments, authors would always reply. I would give them thick pieces of text, or short and sweet little comments on every chapter, and almost 90% of them replied even on a completed fic from five years ago. If the author had moved fandoms, they would still acknowledge the commenter with a little "thank you!" Or a heart or a string of emojis. I was a serial commenter, so not every one I would post on a chapter fic would be replied to but the author would at least acknowledge one.
in the fandom I'm in right now I've gotten Really Into ao3 fic again. I've probably posted fifty comments in the last week and a half, and only one of the authors have consistently replied. In the last two months, it's five authors. that number. I've commented on MINIMUM twenty different authors writing and in the last two months I've had five actual replies. Seven if we include the month prior, but that's skewing it because may I didn't comment As Much.
Comments are not mandatory, from the author, but if someone expresses love and adoration for your thing, I don't know, that seems like something you send love to. I fell victim to that mindset before saying no, actually, I want to share that I liked this comment. Because just like how authors feel alone if something gets so many hits but not any reaction, readers feel disheartened and kind of bad for commenting big long pieces of text and "bothering" the writer. It goes both ways! It's always gone both ways! Be a consistent replier, even just a "<3" or a "✨️" "👀" something. It makes the reader feel better, that they haven't been bothering you by continuously commenting. Even if it's short, I've been putting effort into just sending out my thoughts even if they're garbled because I want the writer to know I loved their work. It makes me feel good, that the shitty comments I leave are not bothering someone who doesn't want to be bothered, but are just vaguely appreciated. I'm going to say, the author doesn't always HAVE to reply, especially on fics with huge numbers of comments. But if you never reply, you're discouraging people from commenting, and understand that vetoes the validity of your complaints about not receiving comments.