Which they shouldn't (when they're not replying to a specific point in a long post, anyway), since we have a perfect archive here. Even if you read a reply to a post 4 year old post written 3 years before you ever first visited the site, the context is still available.
I didn't say it's impossible - but it requires at least one other fetch, more if you want to go up further in the thread. And this is slow. It requires more actions, there's a lot of non-post in a fetch, and it requires you to read the entire post to understand the followup. It just makes it harder to follow a thread, not impossible.