That is just something I have noticed.
I know that NFs are very wide group so I am generalizing since they are NFs that are not like this.
If you attack NFs on their beliefs you will probably provoke a response.
The NFs that is attacked will do anything in its power to prove that the belief system is correct/valid.
However if you do that all the time they will learn how to ignore you.
But that is not the point.
The point is that they will focus on defending their beliefs and they will not start a counter attack on your belief system unless you are really pushing it.
In some cases you will hear a rant or an insult but there will probably be no calm and calculated(really calculated) ofensive.
NFs are also quite unlikey to attack the other persons belief system.
Do you feel uncomfortable doing that or you are afraid that you will be depressed if you do that? Or you are afraid of destroying a friendship (or whatever)?