Because most people know the documentation exists and people responding with RTfM aren’t doing so to be helpful but to be impolite. If you’re gonna respond with RTFM just don’t respond and it’ll have the same effect.
Edit: I feel like I should point out that I agree that RTFM is an incredibly offensive term, but some form of answer like: “it is covered in the documentation” actually conveys one important bit of information: The answer can actually be found in the documentation.
A direct link to the documentation section they should use is the minimal reasonable answer, in my opinion.
Ie, "I looked in the docs and couldn't find how to..."