Hi there. This is something we have considered before, but rejected for a few reasons. I treat it much more in-depth here, but the sum of it is that there are two main reasons. First, very few threads need it. Sometimes no threads do, sometimes one thread does in a day. It is very rare we find ourselves deploying the top level warning in two threads in a day. With 120+ submissions daily, this is less than one percent of threads. So we don't see much utility in putting a warning in all threads when few actually need it. Right now we only drop it in threads that are active, so likely hitting /r/all and/or frontpages, thus getting a lot of non-regulars.

Secondly, as discussed in that link, having an Automod post makes people think that they can respond to that post with the non-rules breaking stuff. People already do when it is done by a mod. We are very much against having a 'quarantined space' where the rules are loosened, and having an automod post of that type would de facto turn into one and likely increase our workload rather than decrease.