Because there are a LOT of users on here that don't have 99,999,999,999,999 posts and have read everything on the boards 50 times over.
Frankly people who whinge about topic's being beaten to death would do better to not comment at all and go about their merry way.
In reality what happens is they say "OMFG D34d H0rs3, i'm so filled with knowledge already that i don't need to read this" and rate down the OP with little or no constructive input to the thread. So people who are asking legit questions often get rated down to hell because senior users feel it is their duty to let everyone know that they've had this discussion before in one of their 99,999,999,999,999 previous posts.
90% of the time people ask questions because they want an answer, not some douchbag bagging them for being curious.
So yeah in short please add "dead horse" to the swear filter. It would be funny, at least, don't you think?