Monks can talk to each other. What you are referring to is monks claiming achievements to lay people.
I don't see anything intrinsically bad in sharing experiences, but there are some possible problems with discussing things with people who you don't know and who are not doing a similar kind of meditation at a similar (or higher) level. I think that some of us who perhaps are doing a few things right don't want to say too much because: (a) we are not real teachers and don't want to mislead others if we are wrong (b) we don't want to get into an embarrassing and possibly demoralising argument (c) we don't want to create anticipation.
In particular, my teachers have often warned me of the pointlessness and dangers (basically (b) above) of discussing experiences with people who have probably not had similar experiences.
My experience with my teachers on a retreat sometimes goes something like:
"I saw X, is that normal?"
"Yes, did you also see Y?"
"Umm, yes, I think so..."
"OK, you're doing fine, try looking for Z next time..."
I.e. basically anything I see they have seen before and have talked to other students about.
I don't have anything like that depth of experience, and certainly not in the variety of approaches that people bring to this board. However, sometimes I see someone asking about something that I think I know about. In that case, unless it's really simple, I'll often try to quote something from a reputable teacher that says what I wanted to say, so they can examine it for themselves and so it's not just my opinion.
There are some here who do have that depth.