AskHistorians is an actively moderated subreddit, which means that moderators routinely remove content that falls outside the sub's purpose (providing in-depth and comprehensive answers to historical questions) or breaks its rules. Particularly for popular threads that get seen by users not familiar with our sub, there can be a lot of comments that get removed for these reasons. Removed comments count towards the visible comment display, but cannot be seen when you click through, resulting in the phenomenon you describe. This is a feature of Reddit's site architecture.

The vast bulk of removed comments are not attempts at answers in the first place, or offer simplified or incorrect responses that do not come anywhere near meeting our requirements for answer quality. Simply put, writing the kind of answer we want to see takes time, and we work to ensure that threads remain free of clutter, jokes and digressions so that when answers do get written, they get the visibility they deserve. Unfortunately, Reddit's algorithms (over which we have no control) tend to mean that new, rising content makes its way into people's feeds, rather than older posts which already have answers. We recommend checking out our Twitter, Facebook, and Sunday Digest feeds to see the (many) questions which do get excellent answers. We also have a Browser Extension that corrects the comment count on threads to actually reflect the number of visible comments, which makes browsing for answers more straightforward.

Some people find r/HistoriansAnswered useful as well as the links crrpit provided.

This is not an uncommon question due to the different way this subreddit is managed, there is also a discussion here on the subject. Hopefully with the links provided (I use the Browser extension and the Sunday Digest as my main ones but different things work for different people), you can find something that works for you in finding threads with actual answers.