Exactly. It’s hard to have a conversation when what was said previously can change. There’s also the problem of people quoting text that ends up being changed subsequently.
The five-minute edit window was selected to try to give a balance between allowing people some time to fix mistakes in their post, while also giving the conversation some permanence. The longer an edit window that’s allowed, the greater the possibility for confusion.
But when all is said and done, it’s only a message board post. Typos shouldn’t be a big deal unless they radically change the meaning of a post. And even if they are, and you miss the edit window, you can post a clarification subsequently.