Because usually:

They have made the same topic as someone else where people have already had a lengthy debate on the subject - restarting a debate is aggravating especially when people have already covered and debated similar concepts just a few days ago in most cases

They don't want to listen to the criticism - if they post a concept it will be broken down by people with more time and knowledge than them and they refuse to acknowledge that these people know more about the game, the lore, or the game engine than them.

They don't balance their idea around all the other jobs in the game or they completely ignore aspects of the lore to try and shoe horn their concept in - Just because its their idea doesn't mean they get to ignore the other jobs in the game and what they bring to the table in terms of utility and numbers, and they can't blatantly ignore aspects of the lore just so their idea has more grounding this later point is less applicable in most cases because most people don't even bother with how the job fits in with the world half the time and try to tack that on at the end which is just lazy.

-

Usually its one of these that sets it off.

At the end of the day all I usually see is people rehashing ideas from other games trying to shoe horn them into FFXIV or its people who want things to change to make a class OP and broken so as far as I am concerned they didn't bother putting that much effort in at all.

I wont respond to this post, it is my opinion and you can disagree with it all you like.