I wouldn't say clients never have clue about requirements. They do have some clues but are unable to express them using the right language and sufficient detail.
Once in a previous life I did process controls. I was given a pipe expander, think a piston with a mushroom head that can expand; the head is slid into a pipe, expanded, and then slid further, expanded....until the pipe had all its joints tested by the expander. I did the controls (motor, sensors, etc.). What I got from our salesman was "deficient", he clearly only knew we could sell such controls. We had meeting the customer's engineers. One fellow there seemed to be the lead.
After pages of notes on what we were to supply, I told him I would write up an operation document and send it to him. He was free to change it in any way he felt necessary. In our next meeting, he had a few corrections but admitted it was well written. My observation was it takes two to tango. If you are producing something blind without customer buy-in on what the thingy is supposed to do, then you will get the impression the customer doesn't have any requirements clue, when in fact you do not have any requirements clue and you produced a widget blindly.
It is not enough to get a requirements document from the customer, you must write the requirements document you require and then ask them to analyze it and makes changes as they see fit. If they shovel in extra crap in that first round, you now have the ammunition to tell them that was not what they originally asked for and you must have another meeting with them. Once they get the idea that these meetings will continue with no actual work being done on their widget, they'll get the idea they should stop asking for extra crap, especially if your side puts extra dollar amounts to that extra crap.
Also, getting the customer to agree on the operation of the widget gets them to think in the level of detail you require and to think about their new widget will operate in edge cases. They become reluctant to ask for changes after that because they have signed up for the operation (so to speak).