To compare every comment on reddit to every other comment in reddit’s entire history would require an index, and if you want to find similar comments instead of exact matches, it becomes a lot harder to do that efficiently. ElasticSearch might be able to do it, but then you need to duplicate all of that data in a separate database and keep it in sync with your main database without affecting performance too much when people are leaving new comments, and that would probably be expensive.
Comparing combinations of comments is probably impossible. Reddit has a massive number of comments to begin with, and the number of possible subtrees of those comments would just be absurd. If you only care about comparing entire threads and not subtrees, then this doesn’t apply, but I don’t know how useful that will be.
Programmers just do what they’re told. If the managers don’t care about something, the programmers won’t work on it.