Because there are (real) energies rooted in the present situation, all most of them do is give you what they perceive to be the most likely outcome based on the energies they pick up at the time. With a totally made up scenario there is nothing to go off or base this on. If it’s someone that doesn’t exist with circumstances that don’t exist there is no “what if” path and no energy to read from.
The real energy to read from is the person asking the question. They are claiming these details to be fact, so the reader is holding that all in their mind as they open themselves to the energies to then ask for guidance or possible pathways for how that scenario could unfold. I imagine that when the reader does this they may be jumping to the most likely alternate/possible path where this fake scenario could exist in that person’s life and then getting details about it. So it basically is reading a “what if” path off of the person that asked the question, even if it’s far off from what’s actually happening in that person’s life and involves people they don’t actually know. That, of course, is assuming that they are locking into any energy at all when doing a reading on a fake scenario. It’s certainly possible in a situation like this that the reader has taken in the details, formed a subconscious opinion of what’s going to happen based on the information given, and mistaken that common-sense subconscious opinion to be psychic information. Maybe they are getting no actual psychic information at all and only picking up their own subconscious bias because the details aren’t real. I'm not sure, just theorizing.
While there are some money-hungry intentional scammers out there doing readings, from what I’ve seen the majority of readers seem to think that doing readings is some type of sacred life path or calling. Given that the majority are not trying to run a con, the details that they get when asked fake questions have to be coming from somewhere.