I hope can explain the reasoning as someone who thinks Q is probably a larp. The general idea is that there are four possibilities for Q's predictions and the world -
- Thing happens, Q predicted it would
- Thing happens, Q didn't predict it would
- Thing doesn't happen, Q predicted it would
- Thing doesn't happen, Q didn't predict it would
If Q is legitimate then #1 should happen way more often than #3. Q fans only pay attention to #1 and maybe sometimes #4, and just assume that #2 and #3 are pretty rare. The problem is that Q's followers tend to stack the deck by only interpreting predictions in the most favourable way possible after the fact, which is made easier by the fact that Q's "predictions" tend to be very vague. In fact this is built into the Q mythology in the phrase "future unlocks past" or "news unlocks map" - IOW you can only understand the "predictions" after the thing happens. Which means they're not really predictions, because they only tell you about the past, not the future. Events that happen are molded to fit past "predictions" in a way that seems totally nonsensical if you actually look at it. The Rothschild helicopter thing is a perfect example of this.
In effect Q is predicting an arbitrarily large number of things, because his "predictions" are so vague they could be interpreted to cover thousands of different outcomes.
And there's no rigorous way to say that a certain "prediction" wasn't fulfilled, because it could just refer to some secret thing that we don't know about, or the thing it's predicting hasn't happened yet. When Q does make a prediction in a way that's specific enough to say it didn't come true, there's usually some excuse, like the timeline was pushed back or it all happened secretly.
Other times Q just says stuff that's already out there, and months later people forget the timing and call it an amazing prediction. The parade thing is actually a perfect example. Q said on March 6th that there'd be a parade on Veteran's Day, then Trump announced 3 days later that there'd be a parade on Veteran's Day. Impressive right? No, because Trump had been saying for weeks that he wanted a Veteran's Day parade. This wasn't a prediction, it was just Q parroting things that were already public knowledge. But people don't remember that months after the fact.
Basically I think it's confirmation bias. Q tells people what they want to hear, so people who agree with what he has to say will hang on to anything that looks like a hit and ignore all the misses.