What I mean is this: a person will have a flashback, and unless the majority of the flashback takes place in front of a mirror, the person remembering should not be able to see themselves in the shot. For these to occur, why don't they film the scene twice, once from a 3rd person perspective, and once from a 1st person perspective?
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chuft probably has the psychology of it nailed, but just another point, from a cinematic perspective...
If the entire scene were shot from the protagonist's PoV, it would start to look very uncomfortable. Firstly, there could be no cuts, as the scene would have to be done in a single shot to maintain the first person perspective. Cuts would feel jumpy, as there would be no clear indication as to why there was a cut. The camera, unless placed so the protagonist was essentially still and facing a single direction for the duration, would have to pan considerably, as the PoV swung to new points of interest. Our eyes do this all the time, but a camera just can't reproduce the effect. It would be the equivalent of following a runner with a hand-held camera [no steadicam] - it works for a short while, but you'd get dizzy and lose the plot after a minute or two.
It would probably be OK if [s]he were looking at events from a distance; shot through a bush etc as though they were peeking at the events, though it wouldn't clearly signify who it was doing the peeking without at least one shot of them doing it, to clarify the position to the audience.
It would look very much stranger if it was a dialogue scene - a conversation between protagonist and others... all the others would be constantly looking right down the barrel of the lens - something which really breaks the immersion of a scene for the audience.
Breaking the fourth wall, as it is called, is usually [sparingly] employed to indicate that the protagonist is speaking directly to the audience, outside of the context of the scene. House of Cards is one of the most famous recent examples of this, though it's by no means a new trick.
I've seen this technique used, though I can't remember the movie. Maybe a Hitchcock or similar. It was done on a wide lens in combination with a [moving?] blur to indicate the person was 'stuck' or 'trapped' and just staring at the ceiling, maybe in a hospital bed with people coming in and out of shot to interact with the patient. iirc, the amount of eye-contact was kept to a minimum... a nurse would say hello, then get on with changing the drip etc.
There is a technique to looking at rather than down the lens, usually done by putting a mark on the edge of the matte-box [the black square surround on the lens] so that the eye-line is just off to one side.
 Late edit
I just recalled, Game of Thrones does this for the wargs' PoV as a wolf etc. Note they don't keep it up for long, as it's quite confusing.
I can't say why things are filmed the way they are. But I can say that a lot of people remember things in a 3rd person perspective, and in fact a lot of people remember some things in a 1st person perspective and other things in a third person perspective. There is a lot of psychological research about this and the factors that influence how people visualize memories. For starters here is the wikipedia entry about it:
I tend to remember things in a third person perspective most of the time, so to me, a flashback should be from the third person perspective, with the recaller seeing themselves in the memory from a distance.