Because as a DPS, it's quite painful to end up in a dungeon where the tank won't turn on their stance or the healer won't Esuna, yet being forced to remain quiet the entire run because even saying "stance please" or "esuna please" will get you GM'd. It's simply too risky to say anything in either scenario (or in others where someone keeps doing something that gets everyone killed, like making a pull they can't handle or doing a mechanic improperly), because they "didn't ask for your advice" and even saying it one time will be deemed "insistent". The only other option is to leave, but I'm trying to NOT get punished, and that's what the 30 minute penalty is made for, to force you to stay regardless of how bad things are going, and praying the other person figures it out on their own.
Even when wiping repeatedly there's pretty much nothing you can say or do to explain what happened, because again, it's rude and actionable no matter what words you use or how politely you try to flower up your language. I can only assume the people who say they never get in trouble for trying to help are running with friends/FCmates/static and not randoms, so of course the chance of being reported is nil. Personally I've never been reported myself, but that's because I refuse to say anything no matter how badly things are going (I just take the leaver penalty), it's simply too dangerous to speak up pretty much anywhere or at anytime.
