The problem is when there starts to be many "site-specific stuff". It's easy to remember not to route .well-known to user-generated content in your app. It's less easy to remember a list of endpoints, like robots.txt, security.txt, and so on. And what when that list grows? What if you already have a user called "security.txt" (or whatever the next one is)? This is why a .well-known prefix is valuable.